Merge tag 'uml-for-linus-6.11-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/uml/linux

Pull UML updates from Richard Weinberger:

 - Support for preemption

 - i386 Rust support

 - Huge cleanup by Benjamin Berg

 - UBSAN support

 - Removal of dead code

* tag 'uml-for-linus-6.11-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/uml/linux: (41 commits)
  um: vector: always reset vp->opened
  um: vector: remove vp->lock
  um: register power-off handler
  um: line: always fill *error_out in setup_one_line()
  um: remove pcap driver from documentation
  um: Enable preemption in UML
  um: refactor TLB update handling
  um: simplify and consolidate TLB updates
  um: remove force_flush_all from fork_handler
  um: Do not flush MM in flush_thread
  um: Delay flushing syscalls until the thread is restarted
  um: remove copy_context_skas0
  um: remove LDT support
  um: compress memory related stub syscalls while adding them
  um: Rework syscall handling
  um: Add generic stub_syscall6 function
  um: Create signal stack memory assignment in stub_data
  um: Remove stub-data.h include from common-offsets.h
  um: time-travel: fix signal blocking race/hang
  um: time-travel: remove time_exit()
  ...

50 files changed, 1043 insertions, 1319 deletions

diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index 93a5a8999b07..dca84fd6d00a 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -11,7 +11,7 @@ config UML
 	select ARCH_HAS_KCOV
 	select ARCH_HAS_STRNCPY_FROM_USER
 	select ARCH_HAS_STRNLEN_USER
-	select ARCH_NO_PREEMPT
+	select ARCH_NO_PREEMPT_DYNAMIC
 	select HAVE_ARCH_AUDITSYSCALL
 	select HAVE_ARCH_KASAN if X86_64
 	select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
@@ -31,7 +31,8 @@ config UML
 	select TRACE_IRQFLAGS_SUPPORT
 	select TTY # Needed for line.c
 	select HAVE_ARCH_VMAP_STACK
-	select HAVE_RUST			if X86_64
+	select HAVE_RUST
+	select ARCH_HAS_UBSAN
 
 config MMU
 	bool
@@ -48,12 +49,13 @@ config NO_IOMEM
 config UML_IOMEM_EMULATION
 	bool
 	select INDIRECT_IOMEM
+	select HAS_IOPORT
 	select GENERIC_PCI_IOMAP
 	select GENERIC_IOMAP
 	select NO_GENERIC_PCI_IOPORT_MAP
 
 config NO_IOPORT_MAP
-	def_bool y
+	def_bool !UML_IOMEM_EMULATION
 
 config ISA
 	bool
diff --git a/arch/um/drivers/Kconfig b/arch/um/drivers/Kconfig
index b94b2618e7d8..ede40a160c5e 100644
--- a/arch/um/drivers/Kconfig
+++ b/arch/um/drivers/Kconfig
@@ -297,26 +297,6 @@ config UML_NET_MCAST
 
 	  If unsure, say N.
 
-config UML_NET_PCAP
-	bool "pcap transport (obsolete)"
-	depends on UML_NET
-	depends on !MODVERSIONS
-	select MAY_HAVE_RUNTIME_DEPS
-	help
-	  The pcap transport makes a pcap packet stream on the host look
-	  like an ethernet device inside UML.  This is useful for making
-	  UML act as a network monitor for the host.  You must have libcap
-	  installed in order to build the pcap transport into UML.
-
-	  For more information, see
-	  <http://user-mode-linux.sourceforge.net/old/networking.html>  That site
-	  has examples of the UML command line to use to enable this option.
-
-	  NOTE: THIS TRANSPORT IS DEPRECATED AND WILL BE REMOVED SOON!!! Please
-	  migrate to UML_NET_VECTOR.
-
-	  If unsure, say N.
-
 config UML_NET_SLIRP
 	bool "SLiRP transport (obsolete)"
 	depends on UML_NET
diff --git a/arch/um/drivers/Makefile b/arch/um/drivers/Makefile
index 0e6af81096fd..57882e6bc215 100644
--- a/arch/um/drivers/Makefile
+++ b/arch/um/drivers/Makefile
@@ -20,14 +20,9 @@ harddog-objs := harddog_kern.o
 harddog-builtin-$(CONFIG_UML_WATCHDOG) := harddog_user.o harddog_user_exp.o
 rtc-objs := rtc_kern.o rtc_user.o
 
-LDFLAGS_pcap.o = $(shell $(CC) $(KBUILD_CFLAGS) -print-file-name=libpcap.a)
-
 LDFLAGS_vde.o = $(shell $(CC) $(CFLAGS) -print-file-name=libvdeplug.a)
 
-targets := pcap_kern.o pcap_user.o vde_kern.o vde_user.o
-
-$(obj)/pcap.o: $(obj)/pcap_kern.o $(obj)/pcap_user.o
-	$(LD) -r -dp -o $@ $^ $(ld_flags)
+targets := vde_kern.o vde_user.o
 
 $(obj)/vde.o: $(obj)/vde_kern.o $(obj)/vde_user.o
 	$(LD) -r -dp -o $@ $^ $(ld_flags)
@@ -49,7 +44,6 @@ obj-$(CONFIG_UML_NET_DAEMON) += daemon.o
 obj-$(CONFIG_UML_NET_VECTOR) += vector.o
 obj-$(CONFIG_UML_NET_VDE) += vde.o
 obj-$(CONFIG_UML_NET_MCAST) += umcast.o
-obj-$(CONFIG_UML_NET_PCAP) += pcap.o
 obj-$(CONFIG_UML_NET) += net.o 
 obj-$(CONFIG_MCONSOLE) += mconsole.o
 obj-$(CONFIG_MMAPPER) += mmapper_kern.o 
@@ -69,7 +63,7 @@ obj-$(CONFIG_UML_RTC) += rtc.o
 obj-$(CONFIG_UML_PCI_OVER_VIRTIO) += virt-pci.o
 
 # pcap_user.o must be added explicitly.
-USER_OBJS := fd.o null.o pty.o tty.o xterm.o slip_common.o pcap_user.o vde_user.o vector_user.o
+USER_OBJS := fd.o null.o pty.o tty.o xterm.o slip_common.o vde_user.o vector_user.o
 CFLAGS_null.o = -DDEV_NULL=$(DEV_NULL_PATH)
 
 CFLAGS_xterm.o += '-DCONFIG_XTERM_CHAN_DEFAULT_EMULATOR="$(CONFIG_XTERM_CHAN_DEFAULT_EMULATOR)"'
diff --git a/arch/um/drivers/chan.h b/arch/um/drivers/chan.h
index e14b9cdf7a33..5a61db512ffb 100644
--- a/arch/um/drivers/chan.h
+++ b/arch/um/drivers/chan.h
@@ -22,7 +22,8 @@ struct chan {
 	unsigned int output:1;
 	unsigned int opened:1;
 	unsigned int enabled:1;
-	int fd;
+	int fd_in;
+	int fd_out; /* only different to fd_in if blocking output is needed */
 	const struct chan_ops *ops;
 	void *data;
 };
diff --git a/arch/um/drivers/chan_kern.c b/arch/um/drivers/chan_kern.c
index 37538b4168da..e78a99816c86 100644
--- a/arch/um/drivers/chan_kern.c
+++ b/arch/um/drivers/chan_kern.c
@@ -81,6 +81,12 @@ static const struct chan_ops not_configged_ops = {
 };
 #endif /* CONFIG_NOCONFIG_CHAN */
 
+static inline bool need_output_blocking(void)
+{
+	return time_travel_mode == TT_MODE_INFCPU ||
+	       time_travel_mode == TT_MODE_EXTERNAL;
+}
+
 static int open_one_chan(struct chan *chan)
 {
 	int fd, err;
@@ -96,15 +102,43 @@ static int open_one_chan(struct chan *chan)
 		return fd;
 
 	err = os_set_fd_block(fd, 0);
-	if (err) {
-		(*chan->ops->close)(fd, chan->data);
-		return err;
-	}
+	if (err)
+		goto out_close;
+
+	chan->fd_in = fd;
+	chan->fd_out = fd;
+
+	/*
+	 * In time-travel modes infinite-CPU and external we need to guarantee
+	 * that any writes to the output succeed immdiately from the point of
+	 * the VM. The best way to do this is to put the FD in blocking mode
+	 * and simply wait/retry until everything is written.
+	 * As every write is guaranteed to complete, we also do not need to
+	 * request an IRQ for the output.
+	 *
+	 * Note that input cannot happen in a time synchronized way. We permit
+	 * it, but time passes very quickly if anything waits for a read.
+	 */
+	if (chan->output && need_output_blocking()) {
+		err = os_dup_file(chan->fd_out);
+		if (err < 0)
+			goto out_close;
 
-	chan->fd = fd;
+		chan->fd_out = err;
+
+		err = os_set_fd_block(chan->fd_out, 1);
+		if (err) {
+			os_close_file(chan->fd_out);
+			goto out_close;
+		}
+	}
 
 	chan->opened = 1;
 	return 0;
+
+out_close:
+	(*chan->ops->close)(fd, chan->data);
+	return err;
 }
 
 static int open_chan(struct list_head *chans)
@@ -125,7 +159,7 @@ static int open_chan(struct list_head *chans)
 void chan_enable_winch(struct chan *chan, struct tty_port *port)
 {
 	if (chan && chan->primary && chan->ops->winch)
-		register_winch(chan->fd, port);
+		register_winch(chan->fd_in, port);
 }
 
 static void line_timer_cb(struct work_struct *work)
@@ -156,8 +190,9 @@ int enable_chan(struct line *line)
 
 		if (chan->enabled)
 			continue;
-		err = line_setup_irq(chan->fd, chan->input, chan->output, line,
-				     chan);
+		err = line_setup_irq(chan->fd_in, chan->input,
+				     chan->output && !need_output_blocking(),
+				     line, chan);
 		if (err)
 			goto out_close;
 
@@ -196,7 +231,8 @@ void free_irqs(void)
 
 		if (chan->input && chan->enabled)
 			um_free_irq(chan->line->read_irq, chan);
-		if (chan->output && chan->enabled)
+		if (chan->output && chan->enabled &&
+		    !need_output_blocking())
 			um_free_irq(chan->line->write_irq, chan);
 		chan->enabled = 0;
 	}
@@ -216,15 +252,19 @@ static void close_one_chan(struct chan *chan, int delay_free_irq)
 	} else {
 		if (chan->input && chan->enabled)
 			um_free_irq(chan->line->read_irq, chan);
-		if (chan->output && chan->enabled)
+		if (chan->output && chan->enabled &&
+		    !need_output_blocking())
 			um_free_irq(chan->line->write_irq, chan);
 		chan->enabled = 0;
 	}
+	if (chan->fd_out != chan->fd_in)
+		os_close_file(chan->fd_out);
 	if (chan->ops->close != NULL)
-		(*chan->ops->close)(chan->fd, chan->data);
+		(*chan->ops->close)(chan->fd_in, chan->data);
 
 	chan->opened = 0;
-	chan->fd = -1;
+	chan->fd_in = -1;
+	chan->fd_out = -1;
 }
 
 void close_chan(struct line *line)
@@ -244,7 +284,7 @@ void close_chan(struct line *line)
 void deactivate_chan(struct chan *chan, int irq)
 {
 	if (chan && chan->enabled)
-		deactivate_fd(chan->fd, irq);
+		deactivate_fd(chan->fd_in, irq);
 }
 
 int write_chan(struct chan *chan, const u8 *buf, size_t len, int write_irq)
@@ -254,7 +294,7 @@ int write_chan(struct chan *chan, const u8 *buf, size_t len, int write_irq)
 	if (len == 0 || !chan || !chan->ops->write)
 		return 0;
 
-	n = chan->ops->write(chan->fd, buf, len, chan->data);
+	n = chan->ops->write(chan->fd_out, buf, len, chan->data);
 	if (chan->primary) {
 		ret = n;
 	}
@@ -268,7 +308,7 @@ int console_write_chan(struct chan *chan, const char *buf, int len)
 	if (!chan || !chan->ops->console_write)
 		return 0;
 
-	n = chan->ops->console_write(chan->fd, buf, len);
+	n = chan->ops->console_write(chan->fd_out, buf, len);
 	if (chan->primary)
 		ret = n;
 	return ret;
@@ -296,14 +336,14 @@ int chan_window_size(struct line *line, unsigned short *rows_out,
 	if (chan && chan->primary) {
 		if (chan->ops->window_size == NULL)
 			return 0;
-		return chan->ops->window_size(chan->fd, chan->data,
+		return chan->ops->window_size(chan->fd_in, chan->data,
 					      rows_out, cols_out);
 	}
 	chan = line->chan_out;
 	if (chan && chan->primary) {
 		if (chan->ops->window_size == NULL)
 			return 0;
-		return chan->ops->window_size(chan->fd, chan->data,
+		return chan->ops->window_size(chan->fd_in, chan->data,
 					      rows_out, cols_out);
 	}
 	return 0;
@@ -319,7 +359,7 @@ static void free_one_chan(struct chan *chan)
 		(*chan->ops->free)(chan->data);
 
 	if (chan->primary && chan->output)
-		ignore_sigio_fd(chan->fd);
+		ignore_sigio_fd(chan->fd_in);
 	kfree(chan);
 }
 
@@ -478,7 +518,8 @@ static struct chan *parse_chan(struct line *line, char *str, int device,
 				 .output 	= 0,
 				 .opened  	= 0,
 				 .enabled  	= 0,
-				 .fd 		= -1,
+				 .fd_in		= -1,
+				 .fd_out	= -1,
 				 .ops 		= ops,
 				 .data 		= data });
 	return chan;
@@ -549,7 +590,7 @@ void chan_interrupt(struct line *line, int irq)
 			schedule_delayed_work(&line->task, 1);
 			goto out;
 		}
-		err = chan->ops->read(chan->fd, &c, chan->data);
+		err = chan->ops->read(chan->fd_in, &c, chan->data);
 		if (err > 0)
 			tty_insert_flip_char(port, c, TTY_NORMAL);
 	} while (err > 0);
diff --git a/arch/um/drivers/chan_user.c b/arch/um/drivers/chan_user.c
index ec04e47b9d79..a66e556012c4 100644
--- a/arch/um/drivers/chan_user.c
+++ b/arch/um/drivers/chan_user.c
@@ -23,7 +23,7 @@ int generic_read(int fd, __u8 *c_out, void *unused)
 {
 	int n;
 
-	n = read(fd, c_out, sizeof(*c_out));
+	CATCH_EINTR(n = read(fd, c_out, sizeof(*c_out)));
 	if (n > 0)
 		return n;
 	else if (n == 0)
@@ -37,11 +37,23 @@ int generic_read(int fd, __u8 *c_out, void *unused)
 
 int generic_write(int fd, const __u8 *buf, size_t n, void *unused)
 {
+	int written = 0;
 	int err;
 
-	err = write(fd, buf, n);
-	if (err > 0)
-		return err;
+	/* The FD may be in blocking mode, as such, need to retry short writes,
+	 * they may have been interrupted by a signal.
+	 */
+	do {
+		errno = 0;
+		err = write(fd, buf + written, n - written);
+		if (err > 0) {
+			written += err;
+			continue;
+		}
+	} while (err < 0 && errno == EINTR);
+
+	if (written > 0)
+		return written;
 	else if (errno == EAGAIN)
 		return 0;
 	else if (err == 0)
diff --git a/arch/um/drivers/harddog_kern.c b/arch/um/drivers/harddog_kern.c
index 60d1c6cab8a9..99a7144b229f 100644
--- a/arch/um/drivers/harddog_kern.c
+++ b/arch/um/drivers/harddog_kern.c
@@ -49,6 +49,7 @@
 #include "mconsole.h"
 #include "harddog.h"
 
+MODULE_DESCRIPTION("UML hardware watchdog");
 MODULE_LICENSE("GPL");
 
 static DEFINE_MUTEX(harddog_mutex);
diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
index d82bc3fdb86e..43d8959cc746 100644
--- a/arch/um/drivers/line.c
+++ b/arch/um/drivers/line.c
@@ -383,6 +383,7 @@ int setup_one_line(struct line *lines, int n, char *init,
 			parse_chan_pair(NULL, line, n, opts, error_out);
 			err = 0;
 		}
+		*error_out = "configured as 'none'";
 	} else {
 		char *new = kstrdup(init, GFP_KERNEL);
 		if (!new) {
@@ -406,6 +407,7 @@ int setup_one_line(struct line *lines, int n, char *init,
 			}
 		}
 		if (err) {
+			*error_out = "failed to parse channel pair";
 			line->init_str = NULL;
 			line->valid = 0;
 			kfree(new);
diff --git a/arch/um/drivers/pcap_kern.c b/arch/um/drivers/pcap_kern.c
deleted file mode 100644
index d9bf95d7867b..000000000000
--- a/arch/um/drivers/pcap_kern.c
+++ /dev/null
@@ -1,113 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-/*
- * Copyright (C) 2002 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
- */
-
-#include <linux/init.h>
-#include <linux/netdevice.h>
-#include <net_kern.h>
-#include "pcap_user.h"
-
-struct pcap_init {
-	char *host_if;
-	int promisc;
-	int optimize;
-	char *filter;
-};
-
-static void pcap_init_kern(struct net_device *dev, void *data)
-{
-	struct uml_net_private *pri;
-	struct pcap_data *ppri;
-	struct pcap_init *init = data;
-
-	pri = netdev_priv(dev);
-	ppri = (struct pcap_data *) pri->user;
-	ppri->host_if = init->host_if;
-	ppri->promisc = init->promisc;
-	ppri->optimize = init->optimize;
-	ppri->filter = init->filter;
-
-	printk("pcap backend, host interface %s\n", ppri->host_if);
-}
-
-static int pcap_read(int fd, struct sk_buff *skb, struct uml_net_private *lp)
-{
-	return pcap_user_read(fd, skb_mac_header(skb),
-			      skb->dev->mtu + ETH_HEADER_OTHER,
-			      (struct pcap_data *) &lp->user);
-}
-
-static int pcap_write(int fd, struct sk_buff *skb, struct uml_net_private *lp)
-{
-	return -EPERM;
-}
-
-static const struct net_kern_info pcap_kern_info = {
-	.init			= pcap_init_kern,
-	.protocol		= eth_protocol,
-	.read			= pcap_read,
-	.write			= pcap_write,
-};
-
-static int pcap_setup(char *str, char **mac_out, void *data)
-{
-	struct pcap_init *init = data;
-	char *remain, *host_if = NULL, *options[2] = { NULL, NULL };
-	int i;
-
-	*init = ((struct pcap_init)
-		{ .host_if 	= "eth0",
-		  .promisc 	= 1,
-		  .optimize 	= 0,
-		  .filter 	= NULL });
-
-	remain = split_if_spec(str, &host_if, &init->filter,
-			       &options[0], &options[1], mac_out, NULL);
-	if (remain != NULL) {
-		printk(KERN_ERR "pcap_setup - Extra garbage on "
-		       "specification : '%s'\n", remain);
-		return 0;
-	}
-
-	if (host_if != NULL)
-		init->host_if = host_if;
-
-	for (i = 0; i < ARRAY_SIZE(options); i++) {
-		if (options[i] == NULL)
-			continue;
-		if (!strcmp(options[i], "promisc"))
-			init->promisc = 1;
-		else if (!strcmp(options[i], "nopromisc"))
-			init->promisc = 0;
-		else if (!strcmp(options[i], "optimize"))
-			init->optimize = 1;
-		else if (!strcmp(options[i], "nooptimize"))
-			init->optimize = 0;
-		else {
-			printk(KERN_ERR "pcap_setup : bad option - '%s'\n",
-			       options[i]);
-			return 0;
-		}
-	}
-
-	return 1;
-}
-
-static struct transport pcap_transport = {
-	.list 		= LIST_HEAD_INIT(pcap_transport.list),
-	.name 		= "pcap",
-	.setup  	= pcap_setup,
-	.user 		= &pcap_user_info,
-	.kern 		= &pcap_kern_info,
-	.private_size 	= sizeof(struct pcap_data),
-	.setup_size 	= sizeof(struct pcap_init),
-};
-
-static int register_pcap(void)
-{
-	register_transport(&pcap_transport);
-	return 0;
-}
-
-late_initcall(register_pcap);
diff --git a/arch/um/drivers/pcap_user.c b/arch/um/drivers/pcap_user.c
deleted file mode 100644
index 52ddda3e3b10..000000000000
--- a/arch/um/drivers/pcap_user.c
+++ /dev/null
@@ -1,137 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-/*
- * Copyright (C) 2002 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
- */
-
-#include <errno.h>
-#include <pcap.h>
-#include <string.h>
-#include <asm/types.h>
-#include <net_user.h>
-#include "pcap_user.h"
-#include <um_malloc.h>
-
-#define PCAP_FD(p) (*(int *)(p))
-
-static int pcap_user_init(void *data, void *dev)
-{
-	struct pcap_data *pri = data;
-	pcap_t *p;
-	char errors[PCAP_ERRBUF_SIZE];
-
-	p = pcap_open_live(pri->host_if, ETH_MAX_PACKET + ETH_HEADER_OTHER,
-			   pri->promisc, 0, errors);
-	if (p == NULL) {
-		printk(UM_KERN_ERR "pcap_user_init : pcap_open_live failed - "
-		       "'%s'\n", errors);
-		return -EINVAL;
-	}
-
-	pri->dev = dev;
-	pri->pcap = p;
-	return 0;
-}
-
-static int pcap_user_open(void *data)
-{
-	struct pcap_data *pri = data;
-	__u32 netmask;
-	int err;
-
-	if (pri->pcap == NULL)
-		return -ENODEV;
-
-	if (pri->filter != NULL) {
-		err = dev_netmask(pri->dev, &netmask);
-		if (err < 0) {
-			printk(UM_KERN_ERR "pcap_user_open : dev_netmask failed\n");
-			return -EIO;
-		}
-
-		pri->compiled = uml_kmalloc(sizeof(struct bpf_program),
-					UM_GFP_KERNEL);
-		if (pri->compiled == NULL) {
-			printk(UM_KERN_ERR "pcap_user_open : kmalloc failed\n");
-			return -ENOMEM;
-		}
-
-		err = pcap_compile(pri->pcap,
-				   (struct bpf_program *) pri->compiled,
-				   pri->filter, pri->optimize, netmask);
-		if (err < 0) {
-			printk(UM_KERN_ERR "pcap_user_open : pcap_compile failed - "
-			       "'%s'\n", pcap_geterr(pri->pcap));
-			goto out;
-		}
-
-		err = pcap_setfilter(pri->pcap, pri->compiled);
-		if (err < 0) {
-			printk(UM_KERN_ERR "pcap_user_open : pcap_setfilter "
-			       "failed - '%s'\n", pcap_geterr(pri->pcap));
-			goto out;
-		}
-	}
-
-	return PCAP_FD(pri->pcap);
-
- out:
-	kfree(pri->compiled);
-	return -EIO;
-}
-
-static void pcap_remove(void *data)
-{
-	struct pcap_data *pri = data;
-
-	if (pri->compiled != NULL)
-		pcap_freecode(pri->compiled);
-
-	if (pri->pcap != NULL)
-		pcap_close(pri->pcap);
-}
-
-struct pcap_handler_data {
-	char *buffer;
-	int len;
-};
-
-static void handler(u_char *data, const struct pcap_pkthdr *header,
-		    const u_char *packet)
-{
-	int len;
-
-	struct pcap_handler_data *hdata = (struct pcap_handler_data *) data;
-
-	len = hdata->len < header->caplen ? hdata->len : header->caplen;
-	memcpy(hdata->buffer, packet, len);
-	hdata->len = len;
-}
-
-int pcap_user_read(int fd, void *buffer, int len, struct pcap_data *pri)
-{
-	struct pcap_handler_data hdata = ((struct pcap_handler_data)
-		                          { .buffer  	= buffer,
-					    .len 	= len });
-	int n;
-
-	n = pcap_dispatch(pri->pcap, 1, handler, (u_char *) &hdata);
-	if (n < 0) {
-		printk(UM_KERN_ERR "pcap_dispatch failed - %s\n",
-		       pcap_geterr(pri->pcap));
-		return -EIO;
-	}
-	else if (n == 0)
-		return 0;
-	return hdata.len;
-}
-
-const struct net_user_info pcap_user_info = {
-	.init		= pcap_user_init,
-	.open		= pcap_user_open,
-	.close	 	= NULL,
-	.remove	 	= pcap_remove,
-	.add_address	= NULL,
-	.delete_address = NULL,
-	.mtu		= ETH_MAX_PACKET,
-	.max_packet	= ETH_MAX_PACKET + ETH_HEADER_OTHER,
-};
diff --git a/arch/um/drivers/pcap_user.h b/arch/um/drivers/pcap_user.h
deleted file mode 100644
index 216246f5f09b..000000000000
--- a/arch/um/drivers/pcap_user.h
+++ /dev/null
@@ -1,21 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0 */
-/* 
- * Copyright (C) 2002 Jeff Dike (jdike@karaya.com)
- */
-
-#include <net_user.h>
-
-struct pcap_data {
-	char *host_if;
-	int promisc;
-	int optimize;
-	char *filter;
-	void *compiled;
-	void *pcap;
-	void *dev;
-};
-
-extern const struct net_user_info pcap_user_info;
-
-extern int pcap_user_read(int fd, void *buf, int len, struct pcap_data *pri);
-
diff --git a/arch/um/drivers/port_kern.c b/arch/um/drivers/port_kern.c
index c52b3ff3c092..a4508470df78 100644
--- a/arch/um/drivers/port_kern.c
+++ b/arch/um/drivers/port_kern.c
@@ -45,15 +45,17 @@ struct connection {
 static irqreturn_t pipe_interrupt(int irq, void *data)
 {
 	struct connection *conn = data;
-	int fd;
+	int n_fds = 1, fd = -1;
+	ssize_t ret;
 
-	fd = os_rcv_fd(conn->socket[0], &conn->helper_pid);
-	if (fd < 0) {
-		if (fd == -EAGAIN)
+	ret = os_rcv_fd_msg(conn->socket[0], &fd, n_fds, &conn->helper_pid,
+			    sizeof(conn->helper_pid));
+	if (ret != sizeof(conn->helper_pid)) {
+		if (ret == -EAGAIN)
 			return IRQ_NONE;
 
-		printk(KERN_ERR "pipe_interrupt : os_rcv_fd returned %d\n",
-		       -fd);
+		printk(KERN_ERR "pipe_interrupt : os_rcv_fd_msg returned %zd\n",
+		       ret);
 		os_close_file(conn->fd);
 	}
 
diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index 9f1e76ddda5a..7f28ec1929dc 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -36,7 +36,6 @@
 #include <linux/vmalloc.h>
 #include <linux/platform_device.h>
 #include <linux/scatterlist.h>
-#include <asm/tlbflush.h>
 #include <kern_util.h>
 #include "mconsole_kern.h"
 #include <init.h>
@@ -106,7 +105,6 @@ static inline void ubd_set_bit(__u64 bit, unsigned char *data)
 #define DRIVER_NAME "uml-blkdev"
 
 static DEFINE_MUTEX(ubd_lock);
-static DEFINE_MUTEX(ubd_mutex); /* replaces BKL, might not be needed */
 
 static int ubd_ioctl(struct block_device *bdev, blk_mode_t mode,
 		     unsigned int cmd, unsigned long arg);
@@ -759,7 +757,6 @@ static int ubd_open_dev(struct ubd *ubd_dev)
 			printk(KERN_ERR "Failed to vmalloc COW bitmap\n");
 			goto error;
 		}
-		flush_tlb_kernel_vm();
 
 		err = read_cow_bitmap(ubd_dev->fd, ubd_dev->cow.bitmap,
 				      ubd_dev->cow.bitmap_offset,
diff --git a/arch/um/drivers/vector_kern.c b/arch/um/drivers/vector_kern.c
index 4279793b11b7..2d473282ab51 100644
--- a/arch/um/drivers/vector_kern.c
+++ b/arch/um/drivers/vector_kern.c
@@ -1115,11 +1115,12 @@ static int irq_rr;
 static int vector_net_close(struct net_device *dev)
 {
 	struct vector_private *vp = netdev_priv(dev);
-	unsigned long flags;
 
 	netif_stop_queue(dev);
 	del_timer(&vp->tl);
 
+	vp->opened = false;
+
 	if (vp->fds == NULL)
 		return 0;
 
@@ -1158,10 +1159,7 @@ static int vector_net_close(struct net_device *dev)
 		destroy_queue(vp->tx_queue);
 	kfree(vp->fds);
 	vp->fds = NULL;
-	spin_lock_irqsave(&vp->lock, flags);
-	vp->opened = false;
 	vp->in_error = false;
-	spin_unlock_irqrestore(&vp->lock, flags);
 	return 0;
 }
 
@@ -1203,17 +1201,12 @@ static void vector_reset_tx(struct work_struct *work)
 static int vector_net_open(struct net_device *dev)
 {
 	struct vector_private *vp = netdev_priv(dev);
-	unsigned long flags;
 	int err = -EINVAL;
 	struct vector_device *vdevice;
 
-	spin_lock_irqsave(&vp->lock, flags);
-	if (vp->opened) {
-		spin_unlock_irqrestore(&vp->lock, flags);
+	if (vp->opened)
 		return -ENXIO;
-	}
 	vp->opened = true;
-	spin_unlock_irqrestore(&vp->lock, flags);
 
 	vp->bpf = uml_vector_user_bpf(get_bpf_file(vp->parsed));
 
@@ -1387,8 +1380,6 @@ static int vector_net_load_bpf_flash(struct net_device *dev,
 		return -1;
 	}
 
-	spin_lock(&vp->lock);
-
 	if (vp->bpf != NULL) {
 		if (vp->opened)
 			uml_vector_detach_bpf(vp->fds->rx_fd, vp->bpf);
@@ -1417,15 +1408,12 @@ static int vector_net_load_bpf_flash(struct net_device *dev,
 	if (vp->opened)
 		result = uml_vector_attach_bpf(vp->fds->rx_fd, vp->bpf);
 
-	spin_unlock(&vp->lock);
-
 	return result;
 
 free_buffer:
 	release_firmware(fw);
 
 flash_fail:
-	spin_unlock(&vp->lock);
 	if (vp->bpf != NULL)
 		kfree(vp->bpf->filter);
 	kfree(vp->bpf);
@@ -1631,7 +1619,6 @@ static void vector_eth_configure(
 	INIT_WORK(&vp->reset_tx, vector_reset_tx);
 
 	timer_setup(&vp->tl, vector_timer_expire, 0);
-	spin_lock_init(&vp->lock);
 
 	/* FIXME */
 	dev->netdev_ops = &vector_netdev_ops;
diff --git a/arch/um/drivers/vector_kern.h b/arch/um/drivers/vector_kern.h
index 2a1fa8e0f3e1..806df551be0b 100644
--- a/arch/um/drivers/vector_kern.h
+++ b/arch/um/drivers/vector_kern.h
@@ -71,7 +71,6 @@ struct vector_estats {
 
 struct vector_private {
 	struct list_head list;
-	spinlock_t lock;
 	struct net_device *dev;
 	struct napi_struct		napi	____cacheline_aligned;
 
diff --git a/arch/um/drivers/xterm.c b/arch/um/drivers/xterm.c
index 6918de5e2956..e4316c7981e8 100644
--- a/arch/um/drivers/xterm.c
+++ b/arch/um/drivers/xterm.c
@@ -156,7 +156,7 @@ static int xterm_open(int input, int output, int primary, void *d,
 	new = xterm_fd(fd, &data->helper_pid);
 	if (new < 0) {
 		err = new;
-		printk(UM_KERN_ERR "xterm_open : os_rcv_fd failed, err = %d\n",
+		printk(UM_KERN_ERR "xterm_open : xterm_fd failed, err = %d\n",
 		       -err);
 		goto out_kill;
 	}
diff --git a/arch/um/drivers/xterm_kern.c b/arch/um/drivers/xterm_kern.c
index 8011e51993d5..3971252cb1a6 100644
--- a/arch/um/drivers/xterm_kern.c
+++ b/arch/um/drivers/xterm_kern.c
@@ -21,12 +21,19 @@ struct xterm_wait {
 static irqreturn_t xterm_interrupt(int irq, void *data)
 {
 	struct xterm_wait *xterm = data;
-	int fd;
+	int fd = -1, n_fds = 1;
+	ssize_t ret;
 
-	fd = os_rcv_fd(xterm->fd, &xterm->pid);
-	if (fd == -EAGAIN)
+	ret = os_rcv_fd_msg(xterm->fd, &fd, n_fds,
+			    &xterm->pid, sizeof(xterm->pid));
+	if (ret == -EAGAIN)
 		return IRQ_NONE;
 
+	if (ret < 0)
+		fd = ret;
+	else if (ret != sizeof(xterm->pid))
+		fd = -EMSGSIZE;
+
 	xterm->new_fd = fd;
 	complete(&xterm->ready);
 
diff --git a/arch/um/include/asm/mmu.h b/arch/um/include/asm/mmu.h
index f2923c767bb9..a3eaca41ff61 100644
--- a/arch/um/include/asm/mmu.h
+++ b/arch/um/include/asm/mmu.h
@@ -7,15 +7,13 @@
 #define __ARCH_UM_MMU_H
 
 #include <mm_id.h>
-#include <asm/mm_context.h>
 
 typedef struct mm_context {
 	struct mm_id id;
-	struct uml_arch_mm_context arch;
-} mm_context_t;
 
-/* Avoid tangled inclusion with asm/ldt.h */
-extern long init_new_ldt(struct mm_context *to_mm, struct mm_context *from_mm);
-extern void free_ldt(struct mm_context *mm);
+	/* Address range in need of a TLB sync */
+	unsigned long sync_tlb_range_from;
+	unsigned long sync_tlb_range_to;
+} mm_context_t;
 
 #endif
diff --git a/arch/um/include/asm/mmu_context.h b/arch/um/include/asm/mmu_context.h
index 68e2eb9cfb47..23dcc914d44e 100644
--- a/arch/um/include/asm/mmu_context.h
+++ b/arch/um/include/asm/mmu_context.h
@@ -13,8 +13,6 @@
 #include <asm/mm_hooks.h>
 #include <asm/mmu.h>
 
-extern void force_flush_all(void);
-
 #define activate_mm activate_mm
 static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
 {
diff --git a/arch/um/include/asm/pgtable.h b/arch/um/include/asm/pgtable.h
index e1ece21dbe3f..5bb397b65efb 100644
--- a/arch/um/include/asm/pgtable.h
+++ b/arch/um/include/asm/pgtable.h
@@ -244,6 +244,38 @@ static inline void set_pte(pte_t *pteptr, pte_t pteval)
 
 #define PFN_PTE_SHIFT		PAGE_SHIFT
 
+static inline void um_tlb_mark_sync(struct mm_struct *mm, unsigned long start,
+				    unsigned long end)
+{
+	if (!mm->context.sync_tlb_range_to) {
+		mm->context.sync_tlb_range_from = start;
+		mm->context.sync_tlb_range_to = end;
+	} else {
+		if (start < mm->context.sync_tlb_range_from)
+			mm->context.sync_tlb_range_from = start;
+		if (end > mm->context.sync_tlb_range_to)
+			mm->context.sync_tlb_range_to = end;
+	}
+}
+
+#define set_ptes set_ptes
+static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
+			    pte_t *ptep, pte_t pte, int nr)
+{
+	/* Basically the default implementation */
+	size_t length = nr * PAGE_SIZE;
+
+	for (;;) {
+		set_pte(ptep, pte);
+		if (--nr == 0)
+			break;
+		ptep++;
+		pte = __pte(pte_val(pte) + (nr << PFN_PTE_SHIFT));
+	}
+
+	um_tlb_mark_sync(mm, addr, addr + length);
+}
+
 #define __HAVE_ARCH_PTE_SAME
 static inline int pte_same(pte_t pte_a, pte_t pte_b)
 {
diff --git a/arch/um/include/asm/tlbflush.h b/arch/um/include/asm/tlbflush.h
index a5bda890390d..db997976b6ea 100644
--- a/arch/um/include/asm/tlbflush.h
+++ b/arch/um/include/asm/tlbflush.h
@@ -9,23 +9,51 @@
 #include <linux/mm.h>
 
 /*
- * TLB flushing:
+ * In UML, we need to sync the TLB over by using mmap/munmap/mprotect syscalls
+ * from the process handling the MM (which can be the kernel itself).
+ *
+ * To track updates, we can hook into set_ptes and flush_tlb_*. With set_ptes
+ * we catch all PTE transitions where memory that was unusable becomes usable.
+ * While with flush_tlb_* we can track any memory that becomes unusable and
+ * even if a higher layer of the page table was modified.
+ *
+ * So, we simply track updates using both methods and mark the memory area to
+ * be synced later on. The only special case is that flush_tlb_kern_* needs to
+ * be executed immediately as there is no good synchronization point in that
+ * case. In contrast, in the set_ptes case we can wait for the next kernel
+ * segfault before we do the synchornization.
  *
- *  - flush_tlb() flushes the current mm struct TLBs
  *  - flush_tlb_all() flushes all processes TLBs
  *  - flush_tlb_mm(mm) flushes the specified mm context TLB's
  *  - flush_tlb_page(vma, vmaddr) flushes one page
- *  - flush_tlb_kernel_vm() flushes the kernel vm area
  *  - flush_tlb_range(vma, start, end) flushes a range of pages
+ *  - flush_tlb_kernel_range(start, end) flushes a range of kernel pages
  */
 
+extern int um_tlb_sync(struct mm_struct *mm);
+
 extern void flush_tlb_all(void);
 extern void flush_tlb_mm(struct mm_struct *mm);
-extern void flush_tlb_range(struct vm_area_struct *vma, unsigned long start, 
-			    unsigned long end);
-extern void flush_tlb_page(struct vm_area_struct *vma, unsigned long address);
-extern void flush_tlb_kernel_vm(void);
-extern void flush_tlb_kernel_range(unsigned long start, unsigned long end);
-extern void __flush_tlb_one(unsigned long addr);
+
+static inline void flush_tlb_page(struct vm_area_struct *vma,
+				  unsigned long address)
+{
+	um_tlb_mark_sync(vma->vm_mm, address, address + PAGE_SIZE);
+}
+
+static inline void flush_tlb_range(struct vm_area_struct *vma,
+				   unsigned long start, unsigned long end)
+{
+	um_tlb_mark_sync(vma->vm_mm, start, end);
+}
+
+static inline void flush_tlb_kernel_range(unsigned long start,
+					  unsigned long end)
+{
+	um_tlb_mark_sync(&init_mm, start, end);
+
+	/* Kernel needs to be synced immediately */
+	um_tlb_sync(&init_mm);
+}
 
 #endif
diff --git a/arch/um/include/shared/as-layout.h b/arch/um/include/shared/as-layout.h
index c22f46a757dc..06292fca5a4d 100644
--- a/arch/um/include/shared/as-layout.h
+++ b/arch/um/include/shared/as-layout.h
@@ -23,7 +23,7 @@
 #define STUB_START stub_start
 #define STUB_CODE STUB_START
 #define STUB_DATA (STUB_CODE + UM_KERN_PAGE_SIZE)
-#define STUB_DATA_PAGES 1 /* must be a power of two */
+#define STUB_DATA_PAGES 2 /* must be a power of two */
 #define STUB_END (STUB_DATA + STUB_DATA_PAGES * UM_KERN_PAGE_SIZE)
 
 #ifndef __ASSEMBLY__
diff --git a/arch/um/include/shared/common-offsets.h b/arch/um/include/shared/common-offsets.h
index 96195483fbd0..579ed946a3a9 100644
--- a/arch/um/include/shared/common-offsets.h
+++ b/arch/um/include/shared/common-offsets.h
@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /* for use by sys-$SUBARCH/kernel-offsets.c */
-#include <stub-data.h>
 
 DEFINE(KERNEL_MADV_REMOVE, MADV_REMOVE);
 
@@ -30,7 +29,3 @@ DEFINE(UML_CONFIG_64BIT, CONFIG_64BIT);
 DEFINE(UML_CONFIG_UML_TIME_TRAVEL_SUPPORT, CONFIG_UML_TIME_TRAVEL_SUPPORT);
 #endif
 
-/* for stub */
-DEFINE(UML_STUB_FIELD_OFFSET, offsetof(struct stub_data, offset));
-DEFINE(UML_STUB_FIELD_CHILD_ERR, offsetof(struct stub_data, child_err));
-DEFINE(UML_STUB_FIELD_FD, offsetof(struct stub_data, fd));
diff --git a/arch/um/include/shared/kern_util.h b/arch/um/include/shared/kern_util.h
index 95521b1f5b20..d8ffd2db168e 100644
--- a/arch/um/include/shared/kern_util.h
+++ b/arch/um/include/shared/kern_util.h
@@ -13,7 +13,6 @@ struct siginfo;
 
 extern int uml_exitcode;
 
-extern int ncpus;
 extern int kmalloc_ok;
 
 #define UML_ROUND_UP(addr) \
diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
index aff8906304ea..9a039d6f1f74 100644
--- a/arch/um/include/shared/os.h
+++ b/arch/um/include/shared/os.h
@@ -163,8 +163,10 @@ extern int os_set_fd_block(int fd, int blocking);
 extern int os_accept_connection(int fd);
 extern int os_create_unix_socket(const char *file, int len, int close_on_exec);
 extern int os_shutdown_socket(int fd, int r, int w);
+extern int os_dup_file(int fd);
 extern void os_close_file(int fd);
-extern int os_rcv_fd(int fd, int *helper_pid_out);
+ssize_t os_rcv_fd_msg(int fd, int *fds, unsigned int n_fds,
+		      void *data, size_t data_len);
 extern int os_connect_socket(const char *name);
 extern int os_file_type(char *file);
 extern int os_file_mode(const char *file, struct openflags *mode_out);
@@ -179,6 +181,8 @@ extern int os_eventfd(unsigned int initval, int flags);
 extern int os_sendmsg_fds(int fd, const void *buf, unsigned int len,
 			  const int *fds, unsigned int fds_num);
 int os_poll(unsigned int n, const int *fds);
+void *os_mmap_rw_shared(int fd, size_t size);
+void *os_mremap_rw_shared(void *old_addr, size_t old_size, size_t new_size);
 
 /* start_up.c */
 extern void os_early_checks(void);
@@ -191,6 +195,9 @@ extern void get_host_cpu_features(
 /* mem.c */
 extern int create_mem_file(unsigned long long len);
 
+/* tlb.c */
+extern void report_enomem(void);
+
 /* process.c */
 extern unsigned long os_process_pc(int pid);
 extern int os_process_parent(int pid);
@@ -268,24 +275,20 @@ extern long long os_persistent_clock_emulation(void);
 extern long long os_nsecs(void);
 
 /* skas/mem.c */
-extern long run_syscall_stub(struct mm_id * mm_idp,
-			     int syscall, unsigned long *args, long expected,
-			     void **addr, int done);
-extern long syscall_stub_data(struct mm_id * mm_idp,
-			      unsigned long *data, int data_count,
-			      void **addr, void **stub_addr);
-extern int map(struct mm_id * mm_idp, unsigned long virt,
-	       unsigned long len, int prot, int phys_fd,
-	       unsigned long long offset, int done, void **data);
-extern int unmap(struct mm_id * mm_idp, unsigned long addr, unsigned long len,
-		 int done, void **data);
-extern int protect(struct mm_id * mm_idp, unsigned long addr,
-		   unsigned long len, unsigned int prot, int done, void **data);
+int syscall_stub_flush(struct mm_id *mm_idp);
+struct stub_syscall *syscall_stub_alloc(struct mm_id *mm_idp);
+void syscall_stub_dump_error(struct mm_id *mm_idp);
+
+int map(struct mm_id *mm_idp, unsigned long virt,
+	unsigned long len, int prot, int phys_fd,
+	unsigned long long offset);
+int unmap(struct mm_id *mm_idp, unsigned long addr, unsigned long len);
+int protect(struct mm_id *mm_idp, unsigned long addr,
+	    unsigned long len, unsigned int prot);
 
 /* skas/process.c */
 extern int is_skas_winch(int pid, int fd, void *data);
 extern int start_userspace(unsigned long stub_stack);
-extern int copy_context_skas0(unsigned long stack, int pid);
 extern void userspace(struct uml_pt_regs *regs, unsigned long *aux_fp_regs);
 extern void new_thread(void *stack, jmp_buf *buf, void (*handler)(void));
 extern void switch_threads(jmp_buf *me, jmp_buf *you);
diff --git a/arch/um/include/shared/skas/mm_id.h b/arch/um/include/shared/skas/mm_id.h
index 92dbf727e384..1e76ba40feba 100644
--- a/arch/um/include/shared/skas/mm_id.h
+++ b/arch/um/include/shared/skas/mm_id.h
@@ -12,7 +12,7 @@ struct mm_id {
 		int pid;
 	} u;
 	unsigned long stack;
-	int kill;
+	int syscall_data_len;
 };
 
 void __switch_mm(struct mm_id *mm_idp);
diff --git a/arch/um/include/shared/skas/skas.h b/arch/um/include/shared/skas/skas.h
index c93d2cbc8f32..ebaa116de30b 100644
--- a/arch/um/include/shared/skas/skas.h
+++ b/arch/um/include/shared/skas/skas.h
@@ -15,5 +15,7 @@ extern void new_thread_handler(void);
 extern void handle_syscall(struct uml_pt_regs *regs);
 extern long execute_syscall_skas(void *r);
 extern unsigned long current_stub_stack(void);
+extern struct mm_id *current_mm_id(void);
+extern void current_mm_sync(void);
 
 #endif
diff --git a/arch/um/include/shared/skas/stub-data.h b/arch/um/include/shared/skas/stub-data.h
index 5e3ade3fb38b..2b6b44759dfa 100644
--- a/arch/um/include/shared/skas/stub-data.h
+++ b/arch/um/include/shared/skas/stub-data.h
@@ -8,10 +8,42 @@
 #ifndef __STUB_DATA_H
 #define __STUB_DATA_H
 
+#include <linux/compiler_types.h>
+#include <as-layout.h>
+#include <sysdep/tls.h>
+
+#define STUB_NEXT_SYSCALL(s) \
+	((struct stub_syscall *) (((unsigned long) s) + (s)->cmd_len))
+
+enum stub_syscall_type {
+	STUB_SYSCALL_UNSET = 0,
+	STUB_SYSCALL_MMAP,
+	STUB_SYSCALL_MUNMAP,
+	STUB_SYSCALL_MPROTECT,
+};
+
+struct stub_syscall {
+	struct {
+		unsigned long addr;
+		unsigned long length;
+		unsigned long offset;
+		int fd;
+		int prot;
+	} mem;
+
+	enum stub_syscall_type syscall;
+};
+
 struct stub_data {
 	unsigned long offset;
-	int fd;
-	long parent_err, child_err;
+	long err, child_err;
+
+	int syscall_data_len;
+	/* 128 leaves enough room for additional fields in the struct */
+	struct stub_syscall syscall_data[(UM_KERN_PAGE_SIZE - 128) / sizeof(struct stub_syscall)] __aligned(16);
+
+	/* Stack for our signal handlers and for calling into . */
+	unsigned char sigstack[UM_KERN_PAGE_SIZE] __aligned(UM_KERN_PAGE_SIZE);
 };
 
 #endif
diff --git a/arch/um/include/shared/timetravel.h b/arch/um/include/shared/timetravel.h
index e5c3d69f1b69..c8db2f213dba 100644
--- a/arch/um/include/shared/timetravel.h
+++ b/arch/um/include/shared/timetravel.h
@@ -15,8 +15,17 @@ enum time_travel_mode {
 #if defined(UML_CONFIG_UML_TIME_TRAVEL_SUPPORT) || \
     defined(CONFIG_UML_TIME_TRAVEL_SUPPORT)
 extern enum time_travel_mode time_travel_mode;
+extern int time_travel_should_print_bc_msg;
 #else
 #define time_travel_mode TT_MODE_OFF
+#define time_travel_should_print_bc_msg 0
 #endif /* (UML_)CONFIG_UML_TIME_TRAVEL_SUPPORT */
 
+void _time_travel_print_bc_msg(void);
+static inline void time_travel_print_bc_msg(void)
+{
+	if (time_travel_should_print_bc_msg)
+		_time_travel_print_bc_msg();
+}
+
 #endif /* _UM_TIME_TRAVEL_H_ */
diff --git a/arch/um/include/shared/user.h b/arch/um/include/shared/user.h
index 326e52450e41..bbab79c0c074 100644
--- a/arch/um/include/shared/user.h
+++ b/arch/um/include/shared/user.h
@@ -42,11 +42,19 @@ extern void panic(const char *fmt, ...)
 #define printk(...) _printk(__VA_ARGS__)
 extern int _printk(const char *fmt, ...)
 	__attribute__ ((format (printf, 1, 2)));
+extern void print_hex_dump(const char *level, const char *prefix_str,
+			   int prefix_type, int rowsize, int groupsize,
+			   const void *buf, size_t len, _Bool ascii);
 #else
 static inline int printk(const char *fmt, ...)
 {
 	return 0;
 }
+static inline void print_hex_dump(const char *level, const char *prefix_str,
+				  int prefix_type, int rowsize, int groupsize,
+				  const void *buf, size_t len, _Bool ascii)
+{
+}
 #endif
 
 extern int in_aton(char *str);
diff --git a/arch/um/kernel/exec.c b/arch/um/kernel/exec.c
index 827a0d3fa589..2c15bb2c104c 100644
--- a/arch/um/kernel/exec.c
+++ b/arch/um/kernel/exec.c
@@ -22,17 +22,8 @@
 
 void flush_thread(void)
 {
-	void *data = NULL;
-	int ret;
-
 	arch_flush_thread(&current->thread.arch);
 
-	ret = unmap(&current->mm->context.id, 0, TASK_SIZE, 1, &data);
-	if (ret) {
-		printk(KERN_ERR "%s - clearing address space failed, err = %d\n",
-		       __func__, ret);
-		force_sig(SIGKILL);
-	}
 	get_safe_registers(current_pt_regs()->regs.gp,
 			   current_pt_regs()->regs.fp);
 
diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
index 635d44606bfe..534e91797f89 100644
--- a/arch/um/kernel/irq.c
+++ b/arch/um/kernel/irq.c
@@ -37,7 +37,7 @@ struct irq_reg {
 	bool pending;
 	bool wakeup;
 #ifdef CONFIG_UML_TIME_TRAVEL_SUPPORT
-	bool pending_on_resume;
+	bool pending_event;
 	void (*timetravel_handler)(int, int, void *,
 				   struct time_travel_event *);
 	struct time_travel_event event;
@@ -56,6 +56,9 @@ static DEFINE_SPINLOCK(irq_lock);
 static LIST_HEAD(active_fds);
 static DECLARE_BITMAP(irqs_allocated, UM_LAST_SIGNAL_IRQ);
 static bool irqs_suspended;
+#ifdef CONFIG_UML_TIME_TRAVEL_SUPPORT
+static bool irqs_pending;
+#endif
 
 static void irq_io_loop(struct irq_reg *irq, struct uml_pt_regs *regs)
 {
@@ -84,9 +87,12 @@ static void irq_event_handler(struct time_travel_event *ev)
 {
 	struct irq_reg *reg = container_of(ev, struct irq_reg, event);
 
-	/* do nothing if suspended - just to cause a wakeup */
-	if (irqs_suspended)
+	/* do nothing if suspended; just cause a wakeup and mark as pending */
+	if (irqs_suspended) {
+		irqs_pending = true;
+		reg->pending_event = true;
 		return;
+	}
 
 	generic_handle_irq(reg->irq);
 }
@@ -110,16 +116,47 @@ static bool irq_do_timetravel_handler(struct irq_entry *entry,
 	if (!reg->event.pending)
 		return false;
 
-	if (irqs_suspended)
-		reg->pending_on_resume = true;
 	return true;
 }
+
+static void irq_do_pending_events(bool timetravel_handlers_only)
+{
+	struct irq_entry *entry;
+
+	if (!irqs_pending || timetravel_handlers_only)
+		return;
+
+	irqs_pending = false;
+
+	list_for_each_entry(entry, &active_fds, list) {
+		enum um_irq_type t;
+
+		for (t = 0; t < NUM_IRQ_TYPES; t++) {
+			struct irq_reg *reg = &entry->reg[t];
+
+			/*
+			 * Any timetravel_handler was invoked already, just
+			 * directly run the IRQ.
+			 */
+			if (reg->pending_event) {
+				irq_enter();
+				generic_handle_irq(reg->irq);
+				irq_exit();
+				reg->pending_event = false;
+			}
+		}
+	}
+}
 #else
 static bool irq_do_timetravel_handler(struct irq_entry *entry,
 				      enum um_irq_type t)
 {
 	return false;
 }
+
+static void irq_do_pending_events(bool timetravel_handlers_only)
+{
+}
 #endif
 
 static void sigio_reg_handler(int idx, struct irq_entry *entry, enum um_irq_type t,
@@ -145,6 +182,8 @@ static void sigio_reg_handler(int idx, struct irq_entry *entry, enum um_irq_type
 	 */
 	if (timetravel_handlers_only) {
 #ifdef CONFIG_UML_TIME_TRAVEL_SUPPORT
+		reg->pending_event = true;
+		irqs_pending = true;
 		mark_sigio_pending();
 #endif
 		return;
@@ -162,6 +201,10 @@ static void _sigio_handler(struct uml_pt_regs *regs,
 	if (timetravel_handlers_only && !um_irq_timetravel_handler_used())
 		return;
 
+	/* Flush out pending events that were ignored due to time-travel. */
+	if (!irqs_suspended)
+		irq_do_pending_events(timetravel_handlers_only);
+
 	while (1) {
 		/* This is now lockless - epoll keeps back-referencesto the irqs
 		 * which have trigger it so there is no need to walk the irq
@@ -195,7 +238,9 @@ static void _sigio_handler(struct uml_pt_regs *regs,
 
 void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs)
 {
+	preempt_disable();
 	_sigio_handler(regs, irqs_suspended);
+	preempt_enable();
 }
 
 static struct irq_entry *get_irq_entry_by_fd(int fd)
@@ -543,30 +588,7 @@ void um_irqs_resume(void)
 	unsigned long flags;
 
 
-	local_irq_save(flags);
-#ifdef CONFIG_UML_TIME_TRAVEL_SUPPORT
-	/*
-	 * We don't need to lock anything here since we're in resume
-	 * and nothing else is running, but have disabled IRQs so we
-	 * don't try anything else with the interrupt list from there.
-	 */
-	list_for_each_entry(entry, &active_fds, list) {
-		enum um_irq_type t;
-
-		for (t = 0; t < NUM_IRQ_TYPES; t++) {
-			struct irq_reg *reg = &entry->reg[t];
-
-			if (reg->pending_on_resume) {
-				irq_enter();
-				generic_handle_irq(reg->irq);
-				irq_exit();
-				reg->pending_on_resume = false;
-			}
-		}
-	}
-#endif
-
-	spin_lock(&irq_lock);
+	spin_lock_irqsave(&irq_lock, flags);
 	list_for_each_entry(entry, &active_fds, list) {
 		if (entry->suspended) {
 			int err = os_set_fd_async(entry->fd);
diff --git a/arch/um/kernel/ksyms.c b/arch/um/kernel/ksyms.c
index 3a85bde3e173..f2fb77da08cf 100644
--- a/arch/um/kernel/ksyms.c
+++ b/arch/um/kernel/ksyms.c
@@ -33,7 +33,7 @@ EXPORT_SYMBOL(os_shutdown_socket);
 EXPORT_SYMBOL(os_create_unix_socket);
 EXPORT_SYMBOL(os_connect_socket);
 EXPORT_SYMBOL(os_accept_connection);
-EXPORT_SYMBOL(os_rcv_fd);
+EXPORT_SYMBOL(os_rcv_fd_msg);
 EXPORT_SYMBOL(run_helper);
 EXPORT_SYMBOL(os_major);
 EXPORT_SYMBOL(os_minor);
diff --git a/arch/um/kernel/mem.c b/arch/um/kernel/mem.c
index ca91accd64fc..a5b4fe2ad931 100644
--- a/arch/um/kernel/mem.c
+++ b/arch/um/kernel/mem.c
@@ -73,7 +73,6 @@ void __init mem_init(void)
 
 	/* this will put all low memory onto the freelists */
 	memblock_free_all();
-	max_low_pfn = totalram_pages();
 	max_pfn = max_low_pfn;
 	kmalloc_ok = 1;
 }
diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index d2134802f6a8..f36b63f53bab 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -122,8 +122,6 @@ void new_thread_handler(void)
 /* Called magically, see new_thread_handler above */
 static void fork_handler(void)
 {
-	force_flush_all();
-
 	schedule_tail(current->thread.prev_sched);
 
 	/*
@@ -237,73 +235,6 @@ int copy_from_user_proc(void *to, void __user *from, int size)
 	return copy_from_user(to, from, size);
 }
 
-static atomic_t using_sysemu = ATOMIC_INIT(0);
-int sysemu_supported;
-
-static void set_using_sysemu(int value)
-{
-	if (value > sysemu_supported)
-		return;
-	atomic_set(&using_sysemu, value);
-}
-
-static int get_using_sysemu(void)
-{
-	return atomic_read(&using_sysemu);
-}
-
-static int sysemu_proc_show(struct seq_file *m, void *v)
-{
-	seq_printf(m, "%d\n", get_using_sysemu());
-	return 0;
-}
-
-static int sysemu_proc_open(struct inode *inode, struct file *file)
-{
-	return single_open(file, sysemu_proc_show, NULL);
-}
-
-static ssize_t sysemu_proc_write(struct file *file, const char __user *buf,
-				 size_t count, loff_t *pos)
-{
-	char tmp[2];
-
-	if (copy_from_user(tmp, buf, 1))
-		return -EFAULT;
-
-	if (tmp[0] >= '0' && tmp[0] <= '2')
-		set_using_sysemu(tmp[0] - '0');
-	/* We use the first char, but pretend to write everything */
-	return count;
-}
-
-static const struct proc_ops sysemu_proc_ops = {
-	.proc_open	= sysemu_proc_open,
-	.proc_read	= seq_read,
-	.proc_lseek	= seq_lseek,
-	.proc_release	= single_release,
-	.proc_write	= sysemu_proc_write,
-};
-
-static int __init make_proc_sysemu(void)
-{
-	struct proc_dir_entry *ent;
-	if (!sysemu_supported)
-		return 0;
-
-	ent = proc_create("sysemu", 0600, NULL, &sysemu_proc_ops);
-
-	if (ent == NULL)
-	{
-		printk(KERN_WARNING "Failed to register /proc/sysemu\n");
-		return 0;
-	}
-
-	return 0;
-}
-
-late_initcall(make_proc_sysemu);
-
 int singlestepping(void)
 {
 	return test_thread_flag(TIF_SINGLESTEP);
diff --git a/arch/um/kernel/reboot.c b/arch/um/kernel/reboot.c
index 25840eee1068..3736bca626ba 100644
--- a/arch/um/kernel/reboot.c
+++ b/arch/um/kernel/reboot.c
@@ -59,3 +59,18 @@ void machine_halt(void)
 {
 	machine_power_off();
 }
+
+static int sys_power_off_handler(struct sys_off_data *data)
+{
+	machine_power_off();
+	return 0;
+}
+
+static int register_power_off(void)
+{
+	register_sys_off_handler(SYS_OFF_MODE_POWER_OFF,
+				 SYS_OFF_PRIO_DEFAULT,
+				 sys_power_off_handler, NULL);
+	return 0;
+}
+__initcall(register_power_off);
diff --git a/arch/um/kernel/skas/Makefile b/arch/um/kernel/skas/Makefile
index f93972a25765..6f86d53e3d69 100644
--- a/arch/um/kernel/skas/Makefile
+++ b/arch/um/kernel/skas/Makefile
@@ -3,15 +3,14 @@
 # Copyright (C) 2002 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
 #
 
-obj-y := clone.o mmu.o process.o syscall.o uaccess.o
+obj-y := stub.o mmu.o process.o syscall.o uaccess.o
 
-# clone.o is in the stub, so it can't be built with profiling
+# stub.o is in the stub, so it can't be built with profiling
 # GCC hardened also auto-enables -fpic, but we need %ebx so it can't work ->
 # disable it
 
-CFLAGS_clone.o := $(CFLAGS_NO_HARDENING)
-UNPROFILE_OBJS := clone.o
-
+CFLAGS_stub.o := $(CFLAGS_NO_HARDENING)
+UNPROFILE_OBJS := stub.o
 KCOV_INSTRUMENT := n
 
 include $(srctree)/arch/um/scripts/Makefile.rules
diff --git a/arch/um/kernel/skas/clone.c b/arch/um/kernel/skas/clone.c
deleted file mode 100644
index 62435187dda4..000000000000
--- a/arch/um/kernel/skas/clone.c
+++ /dev/null
@@ -1,48 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-/*
- * Copyright (C) 2015 Thomas Meyer (thomas@m3y3r.de)
- * Copyright (C) 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
- */
-
-#include <signal.h>
-#include <sched.h>
-#include <asm/unistd.h>
-#include <sys/time.h>
-#include <as-layout.h>
-#include <ptrace_user.h>
-#include <stub-data.h>
-#include <sysdep/stub.h>
-
-/*
- * This is in a separate file because it needs to be compiled with any
- * extraneous gcc flags (-pg, -fprofile-arcs, -ftest-coverage) disabled
- *
- * Use UM_KERN_PAGE_SIZE instead of PAGE_SIZE because that calls getpagesize
- * on some systems.
- */
-
-void __attribute__ ((__section__ (".__syscall_stub")))
-stub_clone_handler(void)
-{
-	struct stub_data *data = get_stub_data();
-	long err;
-
-	err = stub_syscall2(__NR_clone, CLONE_PARENT | CLONE_FILES | SIGCHLD,
-			    (unsigned long)data +
-				STUB_DATA_PAGES * UM_KERN_PAGE_SIZE / 2);
-	if (err) {
-		data->parent_err = err;
-		goto done;
-	}
-
-	err = stub_syscall4(__NR_ptrace, PTRACE_TRACEME, 0, 0, 0);
-	if (err) {
-		data->child_err = err;
-		goto done;
-	}
-
-	remap_stack_and_trap();
-
- done:
-	trap_myself();
-}
diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index aeed1c2aaf3c..47f98d87ea3c 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -14,11 +14,14 @@
 #include <as-layout.h>
 #include <os.h>
 #include <skas.h>
+#include <stub-data.h>
+
+/* Ensure the stub_data struct covers the allocated area */
+static_assert(sizeof(struct stub_data) == STUB_DATA_PAGES * UM_KERN_PAGE_SIZE);
 
 int init_new_context(struct task_struct *task, struct mm_struct *mm)
 {
- 	struct mm_context *from_mm = NULL;
-	struct mm_context *to_mm = &mm->context;
+	struct mm_id *new_id = &mm->context.id;
 	unsigned long stack = 0;
 	int ret = -ENOMEM;
 
@@ -26,34 +29,46 @@ int init_new_context(struct task_struct *task, struct mm_struct *mm)
 	if (stack == 0)
 		goto out;
 
-	to_mm->id.stack = stack;
-	if (current->mm != NULL && current->mm != &init_mm)
-		from_mm = &current->mm->context;
+	new_id->stack = stack;
 
 	block_signals_trace();
-	if (from_mm)
-		to_mm->id.u.pid = copy_context_skas0(stack,
-						     from_mm->id.u.pid);
-	else to_mm->id.u.pid = start_userspace(stack);
+	new_id->u.pid = start_userspace(stack);
 	unblock_signals_trace();
 
-	if (to_mm->id.u.pid < 0) {
-		ret = to_mm->id.u.pid;
+	if (new_id->u.pid < 0) {
+		ret = new_id->u.pid;
 		goto out_free;
 	}
 
-	ret = init_new_ldt(to_mm, from_mm);
-	if (ret < 0) {
-		printk(KERN_ERR "init_new_context_skas - init_ldt"
-		       " failed, errno = %d\n", ret);
-		goto out_free;
-	}
+	/*
+	 * Ensure the new MM is clean and nothing unwanted is mapped.
+	 *
+	 * TODO: We should clear the memory up to STUB_START to ensure there is
+	 * nothing mapped there, i.e. we (currently) have:
+	 *
+	 * |- user memory -|- unused        -|- stub        -|- unused    -|
+	 *                 ^ TASK_SIZE      ^ STUB_START
+	 *
+	 * Meaning we have two unused areas where we may still have valid
+	 * mappings from our internal clone(). That isn't really a problem as
+	 * userspace is not going to access them, but it is definitely not
+	 * correct.
+	 *
+	 * However, we are "lucky" and if rseq is configured, then on 32 bit
+	 * it will fall into the first empty range while on 64 bit it is going
+	 * to use an anonymous mapping in the second range. As such, things
+	 * continue to work for now as long as we don't start unmapping these
+	 * areas.
+	 *
+	 * Change this to STUB_START once we have a clean userspace.
+	 */
+	unmap(new_id, 0, TASK_SIZE);
 
 	return 0;
 
  out_free:
-	if (to_mm->id.stack != 0)
-		free_pages(to_mm->id.stack, ilog2(STUB_DATA_PAGES));
+	if (new_id->stack != 0)
+		free_pages(new_id->stack, ilog2(STUB_DATA_PAGES));
  out:
 	return ret;
 }
@@ -76,5 +91,4 @@ void destroy_context(struct mm_struct *mm)
 	os_kill_ptraced_process(mmu->id.u.pid, 1);
 
 	free_pages(mmu->id.stack, ilog2(STUB_DATA_PAGES));
-	free_ldt(mmu);
 }
diff --git a/arch/um/kernel/skas/process.c b/arch/um/kernel/skas/process.c
index 99a5cbb36083..5f9c1c5f36e2 100644
--- a/arch/um/kernel/skas/process.c
+++ b/arch/um/kernel/skas/process.c
@@ -8,6 +8,8 @@
 #include <linux/sched/task_stack.h>
 #include <linux/sched/task.h>
 
+#include <asm/tlbflush.h>
+
 #include <as-layout.h>
 #include <kern.h>
 #include <os.h>
@@ -50,3 +52,19 @@ unsigned long current_stub_stack(void)
 
 	return current->mm->context.id.stack;
 }
+
+struct mm_id *current_mm_id(void)
+{
+	if (current->mm == NULL)
+		return NULL;
+
+	return &current->mm->context.id;
+}
+
+void current_mm_sync(void)
+{
+	if (current->mm == NULL)
+		return;
+
+	um_tlb_sync(current->mm);
+}
diff --git a/arch/um/kernel/skas/stub.c b/arch/um/kernel/skas/stub.c
new file mode 100644
index 000000000000..5d52ffa682dc
--- /dev/null
+++ b/arch/um/kernel/skas/stub.c
@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2021 Benjamin Berg <benjamin@sipsolutions.net>
+ */
+
+#include <sysdep/stub.h>
+
+static __always_inline int syscall_handler(struct stub_data *d)
+{
+	int i;
+	unsigned long res;
+
+	for (i = 0; i < d->syscall_data_len; i++) {
+		struct stub_syscall *sc = &d->syscall_data[i];
+
+		switch (sc->syscall) {
+		case STUB_SYSCALL_MMAP:
+			res = stub_syscall6(STUB_MMAP_NR,
+					    sc->mem.addr, sc->mem.length,
+					    sc->mem.prot,
+					    MAP_SHARED | MAP_FIXED,
+					    sc->mem.fd, sc->mem.offset);
+			if (res != sc->mem.addr) {
+				d->err = res;
+				d->syscall_data_len = i;
+				return -1;
+			}
+			break;
+		case STUB_SYSCALL_MUNMAP:
+			res = stub_syscall2(__NR_munmap,
+					    sc->mem.addr, sc->mem.length);
+			if (res) {
+				d->err = res;
+				d->syscall_data_len = i;
+				return -1;
+			}
+			break;
+		case STUB_SYSCALL_MPROTECT:
+			res = stub_syscall3(__NR_mprotect,
+					    sc->mem.addr, sc->mem.length,
+					    sc->mem.prot);
+			if (res) {
+				d->err = res;
+				d->syscall_data_len = i;
+				return -1;
+			}
+			break;
+		default:
+			d->err = -95; /* EOPNOTSUPP */
+			d->syscall_data_len = i;
+			return -1;
+		}
+	}
+
+	d->err = 0;
+	d->syscall_data_len = 0;
+
+	return 0;
+}
+
+void __section(".__syscall_stub")
+stub_syscall_handler(void)
+{
+	struct stub_data *d = get_stub_data();
+
+	syscall_handler(d);
+
+	trap_myself();
+}
diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
index a8bfe8be1526..47b9f5e63566 100644
--- a/arch/um/kernel/time.c
+++ b/arch/um/kernel/time.c
@@ -31,6 +31,7 @@ EXPORT_SYMBOL_GPL(time_travel_mode);
 static bool time_travel_start_set;
 static unsigned long long time_travel_start;
 static unsigned long long time_travel_time;
+static unsigned long long time_travel_shm_offset;
 static LIST_HEAD(time_travel_events);
 static LIST_HEAD(time_travel_irqs);
 static unsigned long long time_travel_timer_interval;
@@ -40,8 +41,11 @@ static int time_travel_ext_fd = -1;
 static unsigned int time_travel_ext_waiting;
 static bool time_travel_ext_prev_request_valid;
 static unsigned long long time_travel_ext_prev_request;
-static bool time_travel_ext_free_until_valid;
-static unsigned long long time_travel_ext_free_until;
+static unsigned long long *time_travel_ext_free_until;
+static unsigned long long _time_travel_ext_free_until;
+static u16 time_travel_shm_id;
+static struct um_timetravel_schedshm *time_travel_shm;
+static union um_timetravel_schedshm_client *time_travel_shm_client;
 
 static void time_travel_set_time(unsigned long long ns)
 {
@@ -58,8 +62,52 @@ enum time_travel_message_handling {
 	TTMH_IDLE,
 	TTMH_POLL,
 	TTMH_READ,
+	TTMH_READ_START_ACK,
 };
 
+static u64 bc_message;
+int time_travel_should_print_bc_msg;
+
+void _time_travel_print_bc_msg(void)
+{
+	time_travel_should_print_bc_msg = 0;
+	printk(KERN_INFO "time-travel: received broadcast 0x%llx\n", bc_message);
+}
+
+static void time_travel_setup_shm(int fd, u16 id)
+{
+	u32 len;
+
+	time_travel_shm = os_mmap_rw_shared(fd, sizeof(*time_travel_shm));
+
+	if (!time_travel_shm)
+		goto out;
+
+	len = time_travel_shm->len;
+
+	if (time_travel_shm->version != UM_TIMETRAVEL_SCHEDSHM_VERSION ||
+	    len < struct_size(time_travel_shm, clients, id + 1)) {
+		os_unmap_memory(time_travel_shm, sizeof(*time_travel_shm));
+		time_travel_shm = NULL;
+		goto out;
+	}
+
+	time_travel_shm = os_mremap_rw_shared(time_travel_shm,
+					      sizeof(*time_travel_shm),
+					      len);
+	if (!time_travel_shm)
+		goto out;
+
+	time_travel_shm_offset = time_travel_shm->current_time;
+	time_travel_shm_client = &time_travel_shm->clients[id];
+	time_travel_shm_client->capa |= UM_TIMETRAVEL_SCHEDSHM_CAP_TIME_SHARE;
+	time_travel_shm_id = id;
+	/* always look at that free_until from now on */
+	time_travel_ext_free_until = &time_travel_shm->free_until;
+out:
+	os_close_file(fd);
+}
+
 static void time_travel_handle_message(struct um_timetravel_msg *msg,
 				       enum time_travel_message_handling mode)
 {
@@ -80,7 +128,20 @@ static void time_travel_handle_message(struct um_timetravel_msg *msg,
 		}
 	}
 
-	ret = os_read_file(time_travel_ext_fd, msg, sizeof(*msg));
+	if (unlikely(mode == TTMH_READ_START_ACK)) {
+		int fd[UM_TIMETRAVEL_SHARED_MAX_FDS];
+
+		ret = os_rcv_fd_msg(time_travel_ext_fd, fd,
+				    ARRAY_SIZE(fd), msg, sizeof(*msg));
+		if (ret == sizeof(*msg)) {
+			time_travel_setup_shm(fd[UM_TIMETRAVEL_SHARED_MEMFD],
+					      msg->time & UM_TIMETRAVEL_START_ACK_ID);
+			/* we don't use the logging for now */
+			os_close_file(fd[UM_TIMETRAVEL_SHARED_LOGFD]);
+		}
+	} else {
+		ret = os_read_file(time_travel_ext_fd, msg, sizeof(*msg));
+	}
 
 	if (ret == 0)
 		panic("time-travel external link is broken\n");
@@ -96,10 +157,24 @@ static void time_travel_handle_message(struct um_timetravel_msg *msg,
 		return;
 	case UM_TIMETRAVEL_RUN:
 		time_travel_set_time(msg->time);
+		if (time_travel_shm) {
+			/* no request right now since we're running */
+			time_travel_shm_client->flags &=
+				~UM_TIMETRAVEL_SCHEDSHM_FLAGS_REQ_RUN;
+			/* no ack for shared memory RUN */
+			return;
+		}
 		break;
 	case UM_TIMETRAVEL_FREE_UNTIL:
-		time_travel_ext_free_until_valid = true;
-		time_travel_ext_free_until = msg->time;
+		/* not supposed to get this with shm, but ignore it */
+		if (time_travel_shm)
+			break;
+		time_travel_ext_free_until = &_time_travel_ext_free_until;
+		_time_travel_ext_free_until = msg->time;
+		break;
+	case UM_TIMETRAVEL_BROADCAST:
+		bc_message = msg->time;
+		time_travel_should_print_bc_msg = 1;
 		break;
 	}
 
@@ -136,8 +211,15 @@ static u64 time_travel_ext_req(u32 op, u64 time)
 	block_signals_hard();
 	os_write_file(time_travel_ext_fd, &msg, sizeof(msg));
 
+	/* no ACK expected for WAIT in shared memory mode */
+	if (msg.op == UM_TIMETRAVEL_WAIT && time_travel_shm)
+		goto done;
+
 	while (msg.op != UM_TIMETRAVEL_ACK)
-		time_travel_handle_message(&msg, TTMH_READ);
+		time_travel_handle_message(&msg,
+					   op == UM_TIMETRAVEL_START ?
+						TTMH_READ_START_ACK :
+						TTMH_READ);
 
 	if (msg.seq != mseq)
 		panic("time-travel: ACK message has different seqno! op=%d, seq=%d != %d time=%lld\n",
@@ -145,6 +227,7 @@ static u64 time_travel_ext_req(u32 op, u64 time)
 
 	if (op == UM_TIMETRAVEL_GET)
 		time_travel_set_time(msg.time);
+done:
 	unblock_signals_hard();
 
 	return msg.time;
@@ -180,13 +263,33 @@ static void time_travel_ext_update_request(unsigned long long time)
 	/*
 	 * if we're running and are allowed to run past the request
 	 * then we don't need to update it either
+	 *
+	 * Note for shm we ignore FREE_UNTIL messages and leave the pointer
+	 * to shared memory, and for non-shm the offset is 0.
 	 */
-	if (!time_travel_ext_waiting && time_travel_ext_free_until_valid &&
-	    time < time_travel_ext_free_until)
+	if (!time_travel_ext_waiting && time_travel_ext_free_until &&
+	    time < (*time_travel_ext_free_until - time_travel_shm_offset))
 		return;
 
 	time_travel_ext_prev_request = time;
 	time_travel_ext_prev_request_valid = true;
+
+	if (time_travel_shm) {
+		union um_timetravel_schedshm_client *running;
+
+		running = &time_travel_shm->clients[time_travel_shm->running_id];
+
+		if (running->capa & UM_TIMETRAVEL_SCHEDSHM_CAP_TIME_SHARE) {
+			time_travel_shm_client->flags |=
+				UM_TIMETRAVEL_SCHEDSHM_FLAGS_REQ_RUN;
+			time += time_travel_shm_offset;
+			time_travel_shm_client->req_time = time;
+			if (time < time_travel_shm->free_until)
+				time_travel_shm->free_until = time;
+			return;
+		}
+	}
+
 	time_travel_ext_req(UM_TIMETRAVEL_REQUEST, time);
 }
 
@@ -194,6 +297,14 @@ void __time_travel_propagate_time(void)
 {
 	static unsigned long long last_propagated;
 
+	if (time_travel_shm) {
+		if (time_travel_shm->running_id != time_travel_shm_id)
+			panic("time-travel: setting time while not running\n");
+		time_travel_shm->current_time = time_travel_time +
+						time_travel_shm_offset;
+		return;
+	}
+
 	if (last_propagated == time_travel_time)
 		return;
 
@@ -209,9 +320,12 @@ static bool time_travel_ext_request(unsigned long long time)
 	 * If we received an external sync point ("free until") then we
 	 * don't have to request/wait for anything until then, unless
 	 * we're already waiting.
+	 *
+	 * Note for shm we ignore FREE_UNTIL messages and leave the pointer
+	 * to shared memory, and for non-shm the offset is 0.
 	 */
-	if (!time_travel_ext_waiting && time_travel_ext_free_until_valid &&
-	    time < time_travel_ext_free_until)
+	if (!time_travel_ext_waiting && time_travel_ext_free_until &&
+	    time < (*time_travel_ext_free_until - time_travel_shm_offset))
 		return false;
 
 	time_travel_ext_update_request(time);
@@ -225,7 +339,8 @@ static void time_travel_ext_wait(bool idle)
 	};
 
 	time_travel_ext_prev_request_valid = false;
-	time_travel_ext_free_until_valid = false;
+	if (!time_travel_shm)
+		time_travel_ext_free_until = NULL;
 	time_travel_ext_waiting++;
 
 	time_travel_ext_req(UM_TIMETRAVEL_WAIT, -1);
@@ -248,7 +363,11 @@ static void time_travel_ext_wait(bool idle)
 
 static void time_travel_ext_get_time(void)
 {
-	time_travel_ext_req(UM_TIMETRAVEL_GET, -1);
+	if (time_travel_shm)
+		time_travel_set_time(time_travel_shm->current_time -
+				     time_travel_shm_offset);
+	else
+		time_travel_ext_req(UM_TIMETRAVEL_GET, -1);
 }
 
 static void __time_travel_update_time(unsigned long long ns, bool idle)
@@ -875,9 +994,49 @@ static int setup_time_travel_start(char *str)
 	return 1;
 }
 
-__setup("time-travel-start", setup_time_travel_start);
+__setup("time-travel-start=", setup_time_travel_start);
 __uml_help(setup_time_travel_start,
-"time-travel-start=<seconds>\n"
+"time-travel-start=<nanoseconds>\n"
 "Configure the UML instance's wall clock to start at this value rather than\n"
 "the host's wall clock at the time of UML boot.\n");
+static struct kobject *bc_time_kobject;
+
+static ssize_t bc_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "0x%llx", bc_message);
+}
+
+static ssize_t bc_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	int ret;
+	u64 user_bc_message;
+
+	ret = kstrtou64(buf, 0, &user_bc_message);
+	if (ret)
+		return ret;
+
+	bc_message = user_bc_message;
+
+	time_travel_ext_req(UM_TIMETRAVEL_BROADCAST, bc_message);
+	pr_info("um: time: sent broadcast message: 0x%llx\n", bc_message);
+	return count;
+}
+
+static struct kobj_attribute bc_attribute = __ATTR(bc-message, 0660, bc_show, bc_store);
+
+static int __init um_bc_start(void)
+{
+	if (time_travel_mode != TT_MODE_EXTERNAL)
+		return 0;
+
+	bc_time_kobject = kobject_create_and_add("um-ext-time", kernel_kobj);
+	if (!bc_time_kobject)
+		return 0;
+
+	if (sysfs_create_file(bc_time_kobject, &bc_attribute.attr))
+		pr_debug("failed to create the bc file in /sys/kernel/um_time");
+
+	return 0;
+}
+late_initcall(um_bc_start);
 #endif
diff --git a/arch/um/kernel/tlb.c b/arch/um/kernel/tlb.c
index 8784f03fa4a6..44c6fc697f3a 100644
--- a/arch/um/kernel/tlb.c
+++ b/arch/um/kernel/tlb.c
@@ -15,209 +15,54 @@
 #include <skas.h>
 #include <kern_util.h>
 
-struct host_vm_change {
-	struct host_vm_op {
-		enum { NONE, MMAP, MUNMAP, MPROTECT } type;
-		union {
-			struct {
-				unsigned long addr;
-				unsigned long len;
-				unsigned int prot;
-				int fd;
-				__u64 offset;
-			} mmap;
-			struct {
-				unsigned long addr;
-				unsigned long len;
-			} munmap;
-			struct {
-				unsigned long addr;
-				unsigned long len;
-				unsigned int prot;
-			} mprotect;
-		} u;
-	} ops[1];
-	int userspace;
-	int index;
-	struct mm_struct *mm;
-	void *data;
-	int force;
+struct vm_ops {
+	struct mm_id *mm_idp;
+
+	int (*mmap)(struct mm_id *mm_idp,
+		    unsigned long virt, unsigned long len, int prot,
+		    int phys_fd, unsigned long long offset);
+	int (*unmap)(struct mm_id *mm_idp,
+		     unsigned long virt, unsigned long len);
+	int (*mprotect)(struct mm_id *mm_idp,
+			unsigned long virt, unsigned long len,
+			unsigned int prot);
 };
 
-#define INIT_HVC(mm, force, userspace) \
-	((struct host_vm_change) \
-	 { .ops		= { { .type = NONE } },	\
-	   .mm		= mm, \
-       	   .data	= NULL, \
-	   .userspace	= userspace, \
-	   .index	= 0, \
-	   .force	= force })
-
-static void report_enomem(void)
+static int kern_map(struct mm_id *mm_idp,
+		    unsigned long virt, unsigned long len, int prot,
+		    int phys_fd, unsigned long long offset)
 {
-	printk(KERN_ERR "UML ran out of memory on the host side! "
-			"This can happen due to a memory limitation or "
-			"vm.max_map_count has been reached.\n");
-}
-
-static int do_ops(struct host_vm_change *hvc, int end,
-		  int finished)
-{
-	struct host_vm_op *op;
-	int i, ret = 0;
-
-	for (i = 0; i < end && !ret; i++) {
-		op = &hvc->ops[i];
-		switch (op->type) {
-		case MMAP:
-			if (hvc->userspace)
-				ret = map(&hvc->mm->context.id, op->u.mmap.addr,
-					  op->u.mmap.len, op->u.mmap.prot,
-					  op->u.mmap.fd,
-					  op->u.mmap.offset, finished,
-					  &hvc->data);
-			else
-				map_memory(op->u.mmap.addr, op->u.mmap.offset,
-					   op->u.mmap.len, 1, 1, 1);
-			break;
-		case MUNMAP:
-			if (hvc->userspace)
-				ret = unmap(&hvc->mm->context.id,
-					    op->u.munmap.addr,
-					    op->u.munmap.len, finished,
-					    &hvc->data);
-			else
-				ret = os_unmap_memory(
-					(void *) op->u.munmap.addr,
-						      op->u.munmap.len);
-
-			break;
-		case MPROTECT:
-			if (hvc->userspace)
-				ret = protect(&hvc->mm->context.id,
-					      op->u.mprotect.addr,
-					      op->u.mprotect.len,
-					      op->u.mprotect.prot,
-					      finished, &hvc->data);
-			else
-				ret = os_protect_memory(
-					(void *) op->u.mprotect.addr,
-							op->u.mprotect.len,
-							1, 1, 1);
-			break;
-		default:
-			printk(KERN_ERR "Unknown op type %d in do_ops\n",
-			       op->type);
-			BUG();
-			break;
-		}
-	}
-
-	if (ret == -ENOMEM)
-		report_enomem();
-
-	return ret;
+	/* TODO: Why is executable needed to be always set in the kernel? */
+	return os_map_memory((void *)virt, phys_fd, offset, len,
+			     prot & UM_PROT_READ, prot & UM_PROT_WRITE,
+			     1);
 }
 
-static int add_mmap(unsigned long virt, unsigned long phys, unsigned long len,
-		    unsigned int prot, struct host_vm_change *hvc)
+static int kern_unmap(struct mm_id *mm_idp,
+		      unsigned long virt, unsigned long len)
 {
-	__u64 offset;
-	struct host_vm_op *last;
-	int fd = -1, ret = 0;
-
-	if (hvc->userspace)
-		fd = phys_mapping(phys, &offset);
-	else
-		offset = phys;
-	if (hvc->index != 0) {
-		last = &hvc->ops[hvc->index - 1];
-		if ((last->type == MMAP) &&
-		   (last->u.mmap.addr + last->u.mmap.len == virt) &&
-		   (last->u.mmap.prot == prot) && (last->u.mmap.fd == fd) &&
-		   (last->u.mmap.offset + last->u.mmap.len == offset)) {
-			last->u.mmap.len += len;
-			return 0;
-		}
-	}
-
-	if (hvc->index == ARRAY_SIZE(hvc->ops)) {
-		ret = do_ops(hvc, ARRAY_SIZE(hvc->ops), 0);
-		hvc->index = 0;
-	}
-
-	hvc->ops[hvc->index++] = ((struct host_vm_op)
-				  { .type	= MMAP,
-				    .u = { .mmap = { .addr	= virt,
-						     .len	= len,
-						     .prot	= prot,
-						     .fd	= fd,
-						     .offset	= offset }
-			   } });
-	return ret;
+	return os_unmap_memory((void *)virt, len);
 }
 
-static int add_munmap(unsigned long addr, unsigned long len,
-		      struct host_vm_change *hvc)
+static int kern_mprotect(struct mm_id *mm_idp,
+			 unsigned long virt, unsigned long len,
+			 unsigned int prot)
 {
-	struct host_vm_op *last;
-	int ret = 0;
-
-	if (hvc->index != 0) {
-		last = &hvc->ops[hvc->index - 1];
-		if ((last->type == MUNMAP) &&
-		   (last->u.munmap.addr + last->u.mmap.len == addr)) {
-			last->u.munmap.len += len;
-			return 0;
-		}
-	}
-
-	if (hvc->index == ARRAY_SIZE(hvc->ops)) {
-		ret = do_ops(hvc, ARRAY_SIZE(hvc->ops), 0);
-		hvc->index = 0;
-	}
-
-	hvc->ops[hvc->index++] = ((struct host_vm_op)
-				  { .type	= MUNMAP,
-			     	    .u = { .munmap = { .addr	= addr,
-						       .len	= len } } });
-	return ret;
+	return os_protect_memory((void *)virt, len,
+				 prot & UM_PROT_READ, prot & UM_PROT_WRITE,
+				 1);
 }
 
-static int add_mprotect(unsigned long addr, unsigned long len,
-			unsigned int prot, struct host_vm_change *hvc)
+void report_enomem(void)
 {
-	struct host_vm_op *last;
-	int ret = 0;
-
-	if (hvc->index != 0) {
-		last = &hvc->ops[hvc->index - 1];
-		if ((last->type == MPROTECT) &&
-		   (last->u.mprotect.addr + last->u.mprotect.len == addr) &&
-		   (last->u.mprotect.prot == prot)) {
-			last->u.mprotect.len += len;
-			return 0;
-		}
-	}
-
-	if (hvc->index == ARRAY_SIZE(hvc->ops)) {
-		ret = do_ops(hvc, ARRAY_SIZE(hvc->ops), 0);
-		hvc->index = 0;
-	}
-
-	hvc->ops[hvc->index++] = ((struct host_vm_op)
-				  { .type	= MPROTECT,
-			     	    .u = { .mprotect = { .addr	= addr,
-							 .len	= len,
-							 .prot	= prot } } });
-	return ret;
+	printk(KERN_ERR "UML ran out of memory on the host side! "
+			"This can happen due to a memory limitation or "
+			"vm.max_map_count has been reached.\n");
 }
 
-#define ADD_ROUND(n, inc) (((n) + (inc)) & ~((inc) - 1))
-
 static inline int update_pte_range(pmd_t *pmd, unsigned long addr,
 				   unsigned long end,
-				   struct host_vm_change *hvc)
+				   struct vm_ops *ops)
 {
 	pte_t *pte;
 	int r, w, x, prot, ret = 0;
@@ -235,15 +80,22 @@ static inline int update_pte_range(pmd_t *pmd, unsigned long addr,
 
 		prot = ((r ? UM_PROT_READ : 0) | (w ? UM_PROT_WRITE : 0) |
 			(x ? UM_PROT_EXEC : 0));
-		if (hvc->force || pte_newpage(*pte)) {
+		if (pte_newpage(*pte)) {
 			if (pte_present(*pte)) {
-				if (pte_newpage(*pte))
-					ret = add_mmap(addr, pte_val(*pte) & PAGE_MASK,
-						       PAGE_SIZE, prot, hvc);
+				if (pte_newpage(*pte)) {
+					__u64 offset;
+					unsigned long phys =
+						pte_val(*pte) & PAGE_MASK;
+					int fd =  phys_mapping(phys, &offset);
+
+					ret = ops->mmap(ops->mm_idp, addr,
+							PAGE_SIZE, prot, fd,
+							offset);
+				}
 			} else
-				ret = add_munmap(addr, PAGE_SIZE, hvc);
+				ret = ops->unmap(ops->mm_idp, addr, PAGE_SIZE);
 		} else if (pte_newprot(*pte))
-			ret = add_mprotect(addr, PAGE_SIZE, prot, hvc);
+			ret = ops->mprotect(ops->mm_idp, addr, PAGE_SIZE, prot);
 		*pte = pte_mkuptodate(*pte);
 	} while (pte++, addr += PAGE_SIZE, ((addr < end) && !ret));
 	return ret;
@@ -251,7 +103,7 @@ static inline int update_pte_range(pmd_t *pmd, unsigned long addr,
 
 static inline int update_pmd_range(pud_t *pud, unsigned long addr,
 				   unsigned long end,
-				   struct host_vm_change *hvc)
+				   struct vm_ops *ops)
 {
 	pmd_t *pmd;
 	unsigned long next;
@@ -261,19 +113,20 @@ static inline int update_pmd_range(pud_t *pud, unsigned long addr,
 	do {
 		next = pmd_addr_end(addr, end);
 		if (!pmd_present(*pmd)) {
-			if (hvc->force || pmd_newpage(*pmd)) {
-				ret = add_munmap(addr, next - addr, hvc);
+			if (pmd_newpage(*pmd)) {
+				ret = ops->unmap(ops->mm_idp, addr,
+						 next - addr);
 				pmd_mkuptodate(*pmd);
 			}
 		}
-		else ret = update_pte_range(pmd, addr, next, hvc);
+		else ret = update_pte_range(pmd, addr, next, ops);
 	} while (pmd++, addr = next, ((addr < end) && !ret));
 	return ret;
 }
 
 static inline int update_pud_range(p4d_t *p4d, unsigned long addr,
 				   unsigned long end,
-				   struct host_vm_change *hvc)
+				   struct vm_ops *ops)
 {
 	pud_t *pud;
 	unsigned long next;
@@ -283,19 +136,20 @@ static inline int update_pud_range(p4d_t *p4d, unsigned long addr,
 	do {
 		next = pud_addr_end(addr, end);
 		if (!pud_present(*pud)) {
-			if (hvc->force || pud_newpage(*pud)) {
-				ret = add_munmap(addr, next - addr, hvc);
+			if (pud_newpage(*pud)) {
+				ret = ops->unmap(ops->mm_idp, addr,
+						 next - addr);
 				pud_mkuptodate(*pud);
 			}
 		}
-		else ret = update_pmd_range(pud, addr, next, hvc);
+		else ret = update_pmd_range(pud, addr, next, ops);
 	} while (pud++, addr = next, ((addr < end) && !ret));
 	return ret;
 }
 
 static inline int update_p4d_range(pgd_t *pgd, unsigned long addr,
 				   unsigned long end,
-				   struct host_vm_change *hvc)
+				   struct vm_ops *ops)
 {
 	p4d_t *p4d;
 	unsigned long next;
@@ -305,227 +159,59 @@ static inline int update_p4d_range(pgd_t *pgd, unsigned long addr,
 	do {
 		next = p4d_addr_end(addr, end);
 		if (!p4d_present(*p4d)) {
-			if (hvc->force || p4d_newpage(*p4d)) {
-				ret = add_munmap(addr, next - addr, hvc);
+			if (p4d_newpage(*p4d)) {
+				ret = ops->unmap(ops->mm_idp, addr,
+						 next - addr);
 				p4d_mkuptodate(*p4d);
 			}
 		} else
-			ret = update_pud_range(p4d, addr, next, hvc);
+			ret = update_pud_range(p4d, addr, next, ops);
 	} while (p4d++, addr = next, ((addr < end) && !ret));
 	return ret;
 }
 
-static void fix_range_common(struct mm_struct *mm, unsigned long start_addr,
-			     unsigned long end_addr, int force)
+int um_tlb_sync(struct mm_struct *mm)
 {
 	pgd_t *pgd;
-	struct host_vm_change hvc;
-	unsigned long addr = start_addr, next;
-	int ret = 0, userspace = 1;
+	struct vm_ops ops;
+	unsigned long addr = mm->context.sync_tlb_range_from, next;
+	int ret = 0;
+
+	if (mm->context.sync_tlb_range_to == 0)
+		return 0;
+
+	ops.mm_idp = &mm->context.id;
+	if (mm == &init_mm) {
+		ops.mmap = kern_map;
+		ops.unmap = kern_unmap;
+		ops.mprotect = kern_mprotect;
+	} else {
+		ops.mmap = map;
+		ops.unmap = unmap;
+		ops.mprotect = protect;
+	}
 
-	hvc = INIT_HVC(mm, force, userspace);
 	pgd = pgd_offset(mm, addr);
 	do {
-		next = pgd_addr_end(addr, end_addr);
+		next = pgd_addr_end(addr, mm->context.sync_tlb_range_to);
 		if (!pgd_present(*pgd)) {
-			if (force || pgd_newpage(*pgd)) {
-				ret = add_munmap(addr, next - addr, &hvc);
+			if (pgd_newpage(*pgd)) {
+				ret = ops.unmap(ops.mm_idp, addr,
+						next - addr);
 				pgd_mkuptodate(*pgd);
 			}
 		} else
-			ret = update_p4d_range(pgd, addr, next, &hvc);
-	} while (pgd++, addr = next, ((addr < end_addr) && !ret));
+			ret = update_p4d_range(pgd, addr, next, &ops);
+	} while (pgd++, addr = next,
+		 ((addr < mm->context.sync_tlb_range_to) && !ret));
 
-	if (!ret)
-		ret = do_ops(&hvc, hvc.index, 1);
-
-	/* This is not an else because ret is modified above */
-	if (ret) {
-		struct mm_id *mm_idp = &current->mm->context.id;
-
-		printk(KERN_ERR "fix_range_common: failed, killing current "
-		       "process: %d\n", task_tgid_vnr(current));
-		mm_idp->kill = 1;
-	}
-}
-
-static int flush_tlb_kernel_range_common(unsigned long start, unsigned long end)
-{
-	struct mm_struct *mm;
-	pgd_t *pgd;
-	p4d_t *p4d;
-	pud_t *pud;
-	pmd_t *pmd;
-	pte_t *pte;
-	unsigned long addr, last;
-	int updated = 0, err = 0, force = 0, userspace = 0;
-	struct host_vm_change hvc;
-
-	mm = &init_mm;
-	hvc = INIT_HVC(mm, force, userspace);
-	for (addr = start; addr < end;) {
-		pgd = pgd_offset(mm, addr);
-		if (!pgd_present(*pgd)) {
-			last = ADD_ROUND(addr, PGDIR_SIZE);
-			if (last > end)
-				last = end;
-			if (pgd_newpage(*pgd)) {
-				updated = 1;
-				err = add_munmap(addr, last - addr, &hvc);
-				if (err < 0)
-					panic("munmap failed, errno = %d\n",
-					      -err);
-			}
-			addr = last;
-			continue;
-		}
-
-		p4d = p4d_offset(pgd, addr);
-		if (!p4d_present(*p4d)) {
-			last = ADD_ROUND(addr, P4D_SIZE);
-			if (last > end)
-				last = end;
-			if (p4d_newpage(*p4d)) {
-				updated = 1;
-				err = add_munmap(addr, last - addr, &hvc);
-				if (err < 0)
-					panic("munmap failed, errno = %d\n",
-					      -err);
-			}
-			addr = last;
-			continue;
-		}
-
-		pud = pud_offset(p4d, addr);
-		if (!pud_present(*pud)) {
-			last = ADD_ROUND(addr, PUD_SIZE);
-			if (last > end)
-				last = end;
-			if (pud_newpage(*pud)) {
-				updated = 1;
-				err = add_munmap(addr, last - addr, &hvc);
-				if (err < 0)
-					panic("munmap failed, errno = %d\n",
-					      -err);
-			}
-			addr = last;
-			continue;
-		}
-
-		pmd = pmd_offset(pud, addr);
-		if (!pmd_present(*pmd)) {
-			last = ADD_ROUND(addr, PMD_SIZE);
-			if (last > end)
-				last = end;
-			if (pmd_newpage(*pmd)) {
-				updated = 1;
-				err = add_munmap(addr, last - addr, &hvc);
-				if (err < 0)
-					panic("munmap failed, errno = %d\n",
-					      -err);
-			}
-			addr = last;
-			continue;
-		}
-
-		pte = pte_offset_kernel(pmd, addr);
-		if (!pte_present(*pte) || pte_newpage(*pte)) {
-			updated = 1;
-			err = add_munmap(addr, PAGE_SIZE, &hvc);
-			if (err < 0)
-				panic("munmap failed, errno = %d\n",
-				      -err);
-			if (pte_present(*pte))
-				err = add_mmap(addr, pte_val(*pte) & PAGE_MASK,
-					       PAGE_SIZE, 0, &hvc);
-		}
-		else if (pte_newprot(*pte)) {
-			updated = 1;
-			err = add_mprotect(addr, PAGE_SIZE, 0, &hvc);
-		}
-		addr += PAGE_SIZE;
-	}
-	if (!err)
-		err = do_ops(&hvc, hvc.index, 1);
-
-	if (err < 0)
-		panic("flush_tlb_kernel failed, errno = %d\n", err);
-	return updated;
-}
-
-void flush_tlb_page(struct vm_area_struct *vma, unsigned long address)
-{
-	pgd_t *pgd;
-	p4d_t *p4d;
-	pud_t *pud;
-	pmd_t *pmd;
-	pte_t *pte;
-	struct mm_struct *mm = vma->vm_mm;
-	void *flush = NULL;
-	int r, w, x, prot, err = 0;
-	struct mm_id *mm_id;
-
-	address &= PAGE_MASK;
-
-	pgd = pgd_offset(mm, address);
-	if (!pgd_present(*pgd))
-		goto kill;
-
-	p4d = p4d_offset(pgd, address);
-	if (!p4d_present(*p4d))
-		goto kill;
-
-	pud = pud_offset(p4d, address);
-	if (!pud_present(*pud))
-		goto kill;
-
-	pmd = pmd_offset(pud, address);
-	if (!pmd_present(*pmd))
-		goto kill;
-
-	pte = pte_offset_kernel(pmd, address);
-
-	r = pte_read(*pte);
-	w = pte_write(*pte);
-	x = pte_exec(*pte);
-	if (!pte_young(*pte)) {
-		r = 0;
-		w = 0;
-	} else if (!pte_dirty(*pte)) {
-		w = 0;
-	}
-
-	mm_id = &mm->context.id;
-	prot = ((r ? UM_PROT_READ : 0) | (w ? UM_PROT_WRITE : 0) |
-		(x ? UM_PROT_EXEC : 0));
-	if (pte_newpage(*pte)) {
-		if (pte_present(*pte)) {
-			unsigned long long offset;
-			int fd;
-
-			fd = phys_mapping(pte_val(*pte) & PAGE_MASK, &offset);
-			err = map(mm_id, address, PAGE_SIZE, prot, fd, offset,
-				  1, &flush);
-		}
-		else err = unmap(mm_id, address, PAGE_SIZE, 1, &flush);
-	}
-	else if (pte_newprot(*pte))
-		err = protect(mm_id, address, PAGE_SIZE, prot, 1, &flush);
-
-	if (err) {
-		if (err == -ENOMEM)
-			report_enomem();
-
-		goto kill;
-	}
-
-	*pte = pte_mkuptodate(*pte);
+	if (ret == -ENOMEM)
+		report_enomem();
 
-	return;
+	mm->context.sync_tlb_range_from = 0;
+	mm->context.sync_tlb_range_to = 0;
 
-kill:
-	printk(KERN_ERR "Failed to flush page for address 0x%lx\n", address);
-	force_sig(SIGKILL);
+	return ret;
 }
 
 void flush_tlb_all(void)
@@ -540,60 +226,11 @@ void flush_tlb_all(void)
 	flush_tlb_mm(current->mm);
 }
 
-void flush_tlb_kernel_range(unsigned long start, unsigned long end)
-{
-	flush_tlb_kernel_range_common(start, end);
-}
-
-void flush_tlb_kernel_vm(void)
-{
-	flush_tlb_kernel_range_common(start_vm, end_vm);
-}
-
-void __flush_tlb_one(unsigned long addr)
-{
-	flush_tlb_kernel_range_common(addr, addr + PAGE_SIZE);
-}
-
-static void fix_range(struct mm_struct *mm, unsigned long start_addr,
-		      unsigned long end_addr, int force)
-{
-	/*
-	 * Don't bother flushing if this address space is about to be
-	 * destroyed.
-	 */
-	if (atomic_read(&mm->mm_users) == 0)
-		return;
-
-	fix_range_common(mm, start_addr, end_addr, force);
-}
-
-void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
-		     unsigned long end)
-{
-	if (vma->vm_mm == NULL)
-		flush_tlb_kernel_range_common(start, end);
-	else fix_range(vma->vm_mm, start, end, 0);
-}
-EXPORT_SYMBOL(flush_tlb_range);
-
 void flush_tlb_mm(struct mm_struct *mm)
 {
 	struct vm_area_struct *vma;
 	VMA_ITERATOR(vmi, mm, 0);
 
 	for_each_vma(vmi, vma)
-		fix_range(mm, vma->vm_start, vma->vm_end, 0);
-}
-
-void force_flush_all(void)
-{
-	struct mm_struct *mm = current->mm;
-	struct vm_area_struct *vma;
-	VMA_ITERATOR(vmi, mm, 0);
-
-	mmap_read_lock(mm);
-	for_each_vma(vmi, vma)
-		fix_range(mm, vma->vm_start, vma->vm_end, 1);
-	mmap_read_unlock(mm);
+		um_tlb_mark_sync(mm, vma->vm_start, vma->vm_end);
 }
diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index 6d8ae86ae978..97c8df9c4401 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -113,7 +113,7 @@ good_area:
 #if 0
 	WARN_ON(!pte_young(*pte) || (is_write && !pte_dirty(*pte)));
 #endif
-	flush_tlb_page(vma, address);
+
 out:
 	mmap_read_unlock(mm);
 out_nosemaphore:
@@ -210,8 +210,17 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
 	if (!is_user && regs)
 		current->thread.segv_regs = container_of(regs, struct pt_regs, regs);
 
-	if (!is_user && (address >= start_vm) && (address < end_vm)) {
-		flush_tlb_kernel_vm();
+	if (!is_user && init_mm.context.sync_tlb_range_to) {
+		/*
+		 * Kernel has pending updates from set_ptes that were not
+		 * flushed yet. Syncing them should fix the pagefault (if not
+		 * we'll get here again and panic).
+		 */
+		err = um_tlb_sync(&init_mm);
+		if (err == -ENOMEM)
+			report_enomem();
+		if (err)
+			panic("Failed to sync kernel TLBs: %d", err);
 		goto out;
 	}
 	else if (current->mm == NULL) {
diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c
index e95f805e5004..8e594cda6d77 100644
--- a/arch/um/kernel/um_arch.c
+++ b/arch/um/kernel/um_arch.c
@@ -126,9 +126,6 @@ unsigned long uml_reserved; /* Also modified in mem_init */
 unsigned long start_vm;
 unsigned long end_vm;
 
-/* Set in uml_ncpus_setup */
-int ncpus = 1;
-
 /* Set in early boot */
 static int have_root __initdata;
 static int have_console __initdata;
diff --git a/arch/um/os-Linux/file.c b/arch/um/os-Linux/file.c
index fc4450db59bd..5adf8f630049 100644
--- a/arch/um/os-Linux/file.c
+++ b/arch/um/os-Linux/file.c
@@ -17,6 +17,7 @@
 #include <sys/stat.h>
 #include <sys/sysmacros.h>
 #include <sys/un.h>
+#include <sys/mman.h>
 #include <sys/types.h>
 #include <sys/eventfd.h>
 #include <poll.h>
@@ -240,6 +241,16 @@ out:
 	return err;
 }
 
+int os_dup_file(int fd)
+{
+	int new_fd = dup(fd);
+
+	if (new_fd < 0)
+		return -errno;
+
+	return new_fd;
+}
+
 void os_close_file(int fd)
 {
 	close(fd);
@@ -502,44 +513,47 @@ int os_shutdown_socket(int fd, int r, int w)
 	return 0;
 }
 
-int os_rcv_fd(int fd, int *helper_pid_out)
+/**
+ * os_rcv_fd_msg - receive message with (optional) FDs
+ * @fd: the FD to receive from
+ * @fds: the array for FDs to write to
+ * @n_fds: number of FDs to receive (@fds array size)
+ * @data: the message buffer
+ * @data_len: the size of the message to receive
+ *
+ * Receive a message with FDs.
+ *
+ * Returns: the size of the received message, or an error code
+ */
+ssize_t os_rcv_fd_msg(int fd, int *fds, unsigned int n_fds,
+		      void *data, size_t data_len)
 {
-	int new, n;
-	char buf[CMSG_SPACE(sizeof(new))];
-	struct msghdr msg;
+	char buf[CMSG_SPACE(sizeof(*fds) * n_fds)];
 	struct cmsghdr *cmsg;
-	struct iovec iov;
-
-	msg.msg_name = NULL;
-	msg.msg_namelen = 0;
-	iov = ((struct iovec) { .iov_base  = helper_pid_out,
-				.iov_len   = sizeof(*helper_pid_out) });
-	msg.msg_iov = &iov;
-	msg.msg_iovlen = 1;
-	msg.msg_control = buf;
-	msg.msg_controllen = sizeof(buf);
-	msg.msg_flags = 0;
+	struct iovec iov = {
+		.iov_base = data,
+		.iov_len = data_len,
+	};
+	struct msghdr msg = {
+		.msg_iov = &iov,
+		.msg_iovlen = 1,
+		.msg_control = buf,
+		.msg_controllen = sizeof(buf),
+	};
+	int n;
 
 	n = recvmsg(fd, &msg, 0);
 	if (n < 0)
 		return -errno;
-	else if (n != iov.iov_len)
-		*helper_pid_out = -1;
 
 	cmsg = CMSG_FIRSTHDR(&msg);
-	if (cmsg == NULL) {
-		printk(UM_KERN_ERR "rcv_fd didn't receive anything, "
-		       "error = %d\n", errno);
-		return -1;
-	}
-	if ((cmsg->cmsg_level != SOL_SOCKET) ||
-	    (cmsg->cmsg_type != SCM_RIGHTS)) {
-		printk(UM_KERN_ERR "rcv_fd didn't receive a descriptor\n");
-		return -1;
-	}
+	if (!cmsg ||
+	    cmsg->cmsg_level != SOL_SOCKET ||
+	    cmsg->cmsg_type != SCM_RIGHTS)
+		return n;
 
-	new = ((int *) CMSG_DATA(cmsg))[0];
-	return new;
+	memcpy(fds, CMSG_DATA(cmsg), cmsg->cmsg_len);
+	return n;
 }
 
 int os_create_unix_socket(const char *file, int len, int close_on_exec)
@@ -705,3 +719,25 @@ int os_poll(unsigned int n, const int *fds)
 
 	return -EIO;
 }
+
+void *os_mmap_rw_shared(int fd, size_t size)
+{
+	void *res = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+
+	if (res == MAP_FAILED)
+		return NULL;
+
+	return res;
+}
+
+void *os_mremap_rw_shared(void *old_addr, size_t old_size, size_t new_size)
+{
+	void *res;
+
+	res = mremap(old_addr, old_size, new_size, MREMAP_MAYMOVE, NULL);
+
+	if (res == MAP_FAILED)
+		return NULL;
+
+	return res;
+}
diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index 787cfb9a0308..b11ed66c8bb0 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -8,6 +8,7 @@
 
 #include <stdlib.h>
 #include <stdarg.h>
+#include <stdbool.h>
 #include <errno.h>
 #include <signal.h>
 #include <string.h>
@@ -65,9 +66,7 @@ static void sig_handler_common(int sig, struct siginfo *si, mcontext_t *mc)
 
 int signals_enabled;
 #ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT
-static int signals_blocked;
-#else
-#define signals_blocked 0
+static int signals_blocked, signals_blocked_pending;
 #endif
 static unsigned int signals_pending;
 static unsigned int signals_active = 0;
@@ -76,14 +75,27 @@ static void sig_handler(int sig, struct siginfo *si, mcontext_t *mc)
 {
 	int enabled = signals_enabled;
 
-	if ((signals_blocked || !enabled) && (sig == SIGIO)) {
+#ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT
+	if ((signals_blocked ||
+	     __atomic_load_n(&signals_blocked_pending, __ATOMIC_SEQ_CST)) &&
+	    (sig == SIGIO)) {
+		/* increment so unblock will do another round */
+		__atomic_add_fetch(&signals_blocked_pending, 1,
+				   __ATOMIC_SEQ_CST);
+		return;
+	}
+#endif
+
+	if (!enabled && (sig == SIGIO)) {
 		/*
 		 * In TT_MODE_EXTERNAL, need to still call time-travel
-		 * handlers unless signals are also blocked for the
-		 * external time message processing. This will mark
-		 * signals_pending by itself (only if necessary.)
+		 * handlers. This will mark signals_pending by itself
+		 * (only if necessary.)
+		 * Note we won't get here if signals are hard-blocked
+		 * (which is handled above), in that case the hard-
+		 * unblock will handle things.
 		 */
-		if (!signals_blocked && time_travel_mode == TT_MODE_EXTERNAL)
+		if (time_travel_mode == TT_MODE_EXTERNAL)
 			sigio_run_timetravel_handlers();
 		else
 			signals_pending |= SIGIO_MASK;
@@ -380,33 +392,99 @@ int um_set_signals_trace(int enable)
 #ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT
 void mark_sigio_pending(void)
 {
+	/*
+	 * It would seem that this should be atomic so
+	 * it isn't a read-modify-write with a signal
+	 * that could happen in the middle, losing the
+	 * value set by the signal.
+	 *
+	 * However, this function is only called when in
+	 * time-travel=ext simulation mode, in which case
+	 * the only signal ever pending is SIGIO, which
+	 * is blocked while this can be called, and the
+	 * timer signal (SIGALRM) cannot happen.
+	 */
 	signals_pending |= SIGIO_MASK;
 }
 
 void block_signals_hard(void)
 {
-	if (signals_blocked)
-		return;
-	signals_blocked = 1;
+	signals_blocked++;
 	barrier();
 }
 
 void unblock_signals_hard(void)
 {
+	static bool unblocking;
+
 	if (!signals_blocked)
+		panic("unblocking signals while not blocked");
+
+	if (--signals_blocked)
 		return;
-	/* Must be set to 0 before we check the pending bits etc. */
-	signals_blocked = 0;
+	/*
+	 * Must be set to 0 before we check pending so the
+	 * SIGIO handler will run as normal unless we're still
+	 * going to process signals_blocked_pending.
+	 */
 	barrier();
 
-	if (signals_pending && signals_enabled) {
-		/* this is a bit inefficient, but that's not really important */
-		block_signals();
-		unblock_signals();
-	} else if (signals_pending & SIGIO_MASK) {
-		/* we need to run time-travel handlers even if not enabled */
-		sigio_run_timetravel_handlers();
+	/*
+	 * Note that block_signals_hard()/unblock_signals_hard() can be called
+	 * within the unblock_signals()/sigio_run_timetravel_handlers() below.
+	 * This would still be prone to race conditions since it's actually a
+	 * call _within_ e.g. vu_req_read_message(), where we observed this
+	 * issue, which loops. Thus, if the inner call handles the recorded
+	 * pending signals, we can get out of the inner call with the real
+	 * signal hander no longer blocked, and still have a race. Thus don't
+	 * handle unblocking in the inner call, if it happens, but only in
+	 * the outermost call - 'unblocking' serves as an ownership for the
+	 * signals_blocked_pending decrement.
+	 */
+	if (unblocking)
+		return;
+	unblocking = true;
+
+	while (__atomic_load_n(&signals_blocked_pending, __ATOMIC_SEQ_CST)) {
+		if (signals_enabled) {
+			/* signals are enabled so we can touch this */
+			signals_pending |= SIGIO_MASK;
+			/*
+			 * this is a bit inefficient, but that's
+			 * not really important
+			 */
+			block_signals();
+			unblock_signals();
+		} else {
+			/*
+			 * we need to run time-travel handlers even
+			 * if not enabled
+			 */
+			sigio_run_timetravel_handlers();
+		}
+
+		/*
+		 * The decrement of signals_blocked_pending must be atomic so
+		 * that the signal handler will either happen before or after
+		 * the decrement, not during a read-modify-write:
+		 *  - If it happens before, it can increment it and we'll
+		 *    decrement it and do another round in the loop.
+		 *  - If it happens after it'll see 0 for both signals_blocked
+		 *    and signals_blocked_pending and thus run the handler as
+		 *    usual (subject to signals_enabled, but that's unrelated.)
+		 *
+		 * Note that a call to unblock_signals_hard() within the calls
+		 * to unblock_signals() or sigio_run_timetravel_handlers() above
+		 * will do nothing due to the 'unblocking' state, so this cannot
+		 * underflow as the only one decrementing will be the outermost
+		 * one.
+		 */
+		if (__atomic_sub_fetch(&signals_blocked_pending, 1,
+				       __ATOMIC_SEQ_CST) < 0)
+			panic("signals_blocked_pending underflow");
 	}
+
+	unblocking = false;
 }
 #endif
 
diff --git a/arch/um/os-Linux/skas/mem.c b/arch/um/os-Linux/skas/mem.c
index 1f9c1bffc3a6..c55430775efd 100644
--- a/arch/um/os-Linux/skas/mem.c
+++ b/arch/um/os-Linux/skas/mem.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
+ * Copyright (C) 2021 Benjamin Berg <benjamin@sipsolutions.net>
  * Copyright (C) 2002 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
  */
 
@@ -19,7 +20,30 @@
 #include <sysdep/stub.h>
 #include "../internal.h"
 
-extern char batch_syscall_stub[], __syscall_stub_start[];
+extern char __syscall_stub_start[];
+
+void syscall_stub_dump_error(struct mm_id *mm_idp)
+{
+	struct stub_data *proc_data = (void *)mm_idp->stack;
+	struct stub_syscall *sc;
+
+	if (proc_data->syscall_data_len < 0 ||
+	    proc_data->syscall_data_len >= ARRAY_SIZE(proc_data->syscall_data))
+		panic("Syscall data was corrupted by stub (len is: %d, expected maximum: %d)!",
+			proc_data->syscall_data_len,
+			mm_idp->syscall_data_len);
+
+	sc = &proc_data->syscall_data[proc_data->syscall_data_len];
+
+	printk(UM_KERN_ERR "%s : length = %d, last offset = %d",
+		__func__, mm_idp->syscall_data_len,
+		proc_data->syscall_data_len);
+	printk(UM_KERN_ERR "%s : stub syscall type %d failed, return value = 0x%lx\n",
+		__func__, sc->syscall, proc_data->err);
+
+	print_hex_dump(UM_KERN_ERR, "    syscall data: ", 0,
+		       16, 4, sc, sizeof(*sc), 0);
+}
 
 static inline unsigned long *check_init_stack(struct mm_id * mm_idp,
 					      unsigned long *stack)
@@ -36,22 +60,24 @@ static unsigned long syscall_regs[MAX_REG_NR];
 static int __init init_syscall_regs(void)
 {
 	get_safe_registers(syscall_regs, NULL);
+
 	syscall_regs[REGS_IP_INDEX] = STUB_CODE +
-		((unsigned long) batch_syscall_stub -
+		((unsigned long) stub_syscall_handler -
 		 (unsigned long) __syscall_stub_start);
-	syscall_regs[REGS_SP_INDEX] = STUB_DATA;
+	syscall_regs[REGS_SP_INDEX] = STUB_DATA +
+		offsetof(struct stub_data, sigstack) +
+		sizeof(((struct stub_data *) 0)->sigstack) -
+		sizeof(void *);
 
 	return 0;
 }
 
 __initcall(init_syscall_regs);
 
-static inline long do_syscall_stub(struct mm_id * mm_idp, void **addr)
+static inline long do_syscall_stub(struct mm_id *mm_idp)
 {
+	struct stub_data *proc_data = (void *)mm_idp->stack;
 	int n, i;
-	long ret, offset;
-	unsigned long * data;
-	unsigned long * syscall;
 	int err, pid = mm_idp->u.pid;
 
 	n = ptrace_setregs(pid, syscall_regs);
@@ -63,6 +89,9 @@ static inline long do_syscall_stub(struct mm_id * mm_idp, void **addr)
 		      __func__, -n);
 	}
 
+	/* Inform process how much we have filled in. */
+	proc_data->syscall_data_len = mm_idp->syscall_data_len;
+
 	err = ptrace(PTRACE_CONT, pid, 0, 0);
 	if (err)
 		panic("Failed to continue stub, pid = %d, errno = %d\n", pid,
@@ -71,135 +100,141 @@ static inline long do_syscall_stub(struct mm_id * mm_idp, void **addr)
 	wait_stub_done(pid);
 
 	/*
-	 * When the stub stops, we find the following values on the
-	 * beginning of the stack:
-	 * (long )return_value
-	 * (long )offset to failed sycall-data (0, if no error)
+	 * proc_data->err will be non-zero if there was an (unexpected) error.
+	 * In that case, syscall_data_len points to the last executed syscall,
+	 * otherwise it will be zero (but we do not need to rely on that).
 	 */
-	ret = *((unsigned long *) mm_idp->stack);
-	offset = *((unsigned long *) mm_idp->stack + 1);
-	if (offset) {
-		data = (unsigned long *)(mm_idp->stack + offset - STUB_DATA);
-		printk(UM_KERN_ERR "%s : ret = %ld, offset = %ld, data = %p\n",
-		       __func__, ret, offset, data);
-		syscall = (unsigned long *)((unsigned long)data + data[0]);
-		printk(UM_KERN_ERR "%s: syscall %ld failed, return value = 0x%lx, expected return value = 0x%lx\n",
-		       __func__, syscall[0], ret, syscall[7]);
-		printk(UM_KERN_ERR "    syscall parameters: 0x%lx 0x%lx 0x%lx 0x%lx 0x%lx 0x%lx\n",
-		       syscall[1], syscall[2], syscall[3],
-		       syscall[4], syscall[5], syscall[6]);
-		for (n = 1; n < data[0]/sizeof(long); n++) {
-			if (n == 1)
-				printk(UM_KERN_ERR "    additional syscall data:");
-			if (n % 4 == 1)
-				printk("\n" UM_KERN_ERR "      ");
-			printk("  0x%lx", data[n]);
-		}
-		if (n > 1)
-			printk("\n");
-	}
-	else ret = 0;
+	if (proc_data->err < 0) {
+		syscall_stub_dump_error(mm_idp);
 
-	*addr = check_init_stack(mm_idp, NULL);
+		/* Store error code in case someone tries to add more syscalls */
+		mm_idp->syscall_data_len = proc_data->err;
+	} else {
+		mm_idp->syscall_data_len = 0;
+	}
 
-	return ret;
+	return mm_idp->syscall_data_len;
 }
 
-long run_syscall_stub(struct mm_id * mm_idp, int syscall,
-		      unsigned long *args, long expected, void **addr,
-		      int done)
+int syscall_stub_flush(struct mm_id *mm_idp)
 {
-	unsigned long *stack = check_init_stack(mm_idp, *addr);
-
-	*stack += sizeof(long);
-	stack += *stack / sizeof(long);
-
-	*stack++ = syscall;
-	*stack++ = args[0];
-	*stack++ = args[1];
-	*stack++ = args[2];
-	*stack++ = args[3];
-	*stack++ = args[4];
-	*stack++ = args[5];
-	*stack++ = expected;
-	*stack = 0;
-
-	if (!done && ((((unsigned long) stack) & ~UM_KERN_PAGE_MASK) <
-		     UM_KERN_PAGE_SIZE - 10 * sizeof(long))) {
-		*addr = stack;
+	int res;
+
+	if (mm_idp->syscall_data_len == 0)
 		return 0;
+
+	/* If an error happened already, report it and reset the state. */
+	if (mm_idp->syscall_data_len < 0) {
+		res = mm_idp->syscall_data_len;
+		mm_idp->syscall_data_len = 0;
+		return res;
 	}
 
-	return do_syscall_stub(mm_idp, addr);
+	res = do_syscall_stub(mm_idp);
+	mm_idp->syscall_data_len = 0;
+
+	return res;
 }
 
-long syscall_stub_data(struct mm_id * mm_idp,
-		       unsigned long *data, int data_count,
-		       void **addr, void **stub_addr)
+struct stub_syscall *syscall_stub_alloc(struct mm_id *mm_idp)
 {
-	unsigned long *stack;
-	int ret = 0;
-
-	/*
-	 * If *addr still is uninitialized, it *must* contain NULL.
-	 * Thus in this case do_syscall_stub correctly won't be called.
-	 */
-	if ((((unsigned long) *addr) & ~UM_KERN_PAGE_MASK) >=
-	   UM_KERN_PAGE_SIZE - (10 + data_count) * sizeof(long)) {
-		ret = do_syscall_stub(mm_idp, addr);
-		/* in case of error, don't overwrite data on stack */
-		if (ret)
-			return ret;
+	struct stub_syscall *sc;
+	struct stub_data *proc_data = (struct stub_data *) mm_idp->stack;
+
+	if (mm_idp->syscall_data_len > 0 &&
+	    mm_idp->syscall_data_len == ARRAY_SIZE(proc_data->syscall_data))
+		do_syscall_stub(mm_idp);
+
+	if (mm_idp->syscall_data_len < 0) {
+		/* Return dummy to retain error state. */
+		sc = &proc_data->syscall_data[0];
+	} else {
+		sc = &proc_data->syscall_data[mm_idp->syscall_data_len];
+		mm_idp->syscall_data_len += 1;
 	}
+	memset(sc, 0, sizeof(*sc));
 
-	stack = check_init_stack(mm_idp, *addr);
-	*addr = stack;
+	return sc;
+}
 
-	*stack = data_count * sizeof(long);
+static struct stub_syscall *syscall_stub_get_previous(struct mm_id *mm_idp,
+						      int syscall_type,
+						      unsigned long virt)
+{
+	if (mm_idp->syscall_data_len > 0) {
+		struct stub_data *proc_data = (void *) mm_idp->stack;
+		struct stub_syscall *sc;
 
-	memcpy(stack + 1, data, data_count * sizeof(long));
+		sc = &proc_data->syscall_data[mm_idp->syscall_data_len - 1];
 
-	*stub_addr = (void *)(((unsigned long)(stack + 1) &
-			       ~UM_KERN_PAGE_MASK) + STUB_DATA);
+		if (sc->syscall == syscall_type &&
+		    sc->mem.addr + sc->mem.length == virt)
+			return sc;
+	}
 
-	return 0;
+	return NULL;
 }
 
-int map(struct mm_id * mm_idp, unsigned long virt, unsigned long len, int prot,
-	int phys_fd, unsigned long long offset, int done, void **data)
+int map(struct mm_id *mm_idp, unsigned long virt, unsigned long len, int prot,
+	int phys_fd, unsigned long long offset)
 {
-	int ret;
-	unsigned long args[] = { virt, len, prot,
-				 MAP_SHARED | MAP_FIXED, phys_fd,
-				 MMAP_OFFSET(offset) };
+	struct stub_syscall *sc;
 
-	ret = run_syscall_stub(mm_idp, STUB_MMAP_NR, args, virt,
-			       data, done);
+	/* Compress with previous syscall if that is possible */
+	sc = syscall_stub_get_previous(mm_idp, STUB_SYSCALL_MMAP, virt);
+	if (sc && sc->mem.prot == prot && sc->mem.fd == phys_fd &&
+	    sc->mem.offset == MMAP_OFFSET(offset - sc->mem.length)) {
+		sc->mem.length += len;
+		return 0;
+	}
 
-	return ret;
+	sc = syscall_stub_alloc(mm_idp);
+	sc->syscall = STUB_SYSCALL_MMAP;
+	sc->mem.addr = virt;
+	sc->mem.length = len;
+	sc->mem.prot = prot;
+	sc->mem.fd = phys_fd;
+	sc->mem.offset = MMAP_OFFSET(offset);
+
+	return 0;
 }
 
-int unmap(struct mm_id * mm_idp, unsigned long addr, unsigned long len,
-	  int done, void **data)
+int unmap(struct mm_id *mm_idp, unsigned long addr, unsigned long len)
 {
-	int ret;
-	unsigned long args[] = { (unsigned long) addr, len, 0, 0, 0,
-				 0 };
+	struct stub_syscall *sc;
 
-	ret = run_syscall_stub(mm_idp, __NR_munmap, args, 0,
-			       data, done);
+	/* Compress with previous syscall if that is possible */
+	sc = syscall_stub_get_previous(mm_idp, STUB_SYSCALL_MUNMAP, addr);
+	if (sc) {
+		sc->mem.length += len;
+		return 0;
+	}
 
-	return ret;
+	sc = syscall_stub_alloc(mm_idp);
+	sc->syscall = STUB_SYSCALL_MUNMAP;
+	sc->mem.addr = addr;
+	sc->mem.length = len;
+
+	return 0;
 }
 
-int protect(struct mm_id * mm_idp, unsigned long addr, unsigned long len,
-	    unsigned int prot, int done, void **data)
+int protect(struct mm_id *mm_idp, unsigned long addr, unsigned long len,
+	    unsigned int prot)
 {
-	int ret;
-	unsigned long args[] = { addr, len, prot, 0, 0, 0 };
+	struct stub_syscall *sc;
 
-	ret = run_syscall_stub(mm_idp, __NR_mprotect, args, 0,
-			       data, done);
+	/* Compress with previous syscall if that is possible */
+	sc = syscall_stub_get_previous(mm_idp, STUB_SYSCALL_MPROTECT, addr);
+	if (sc && sc->mem.prot == prot) {
+		sc->mem.length += len;
+		return 0;
+	}
 
-	return ret;
+	sc = syscall_stub_alloc(mm_idp);
+	sc->syscall = STUB_SYSCALL_MPROTECT;
+	sc->mem.addr = addr;
+	sc->mem.length = len;
+	sc->mem.prot = prot;
+
+	return 0;
 }
diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 41a288dcfc34..f7088345b3fc 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -23,6 +23,7 @@
 #include <skas.h>
 #include <sysdep/stub.h>
 #include <linux/threads.h>
+#include <timetravel.h>
 #include "../internal.h"
 
 int is_skas_winch(int pid, int fd, void *data)
@@ -253,7 +254,6 @@ static int userspace_tramp(void *stack)
 }
 
 int userspace_pid[NR_CPUS];
-int kill_userspace_mm[NR_CPUS];
 
 /**
  * start_userspace() - prepare a new userspace process
@@ -345,8 +345,20 @@ void userspace(struct uml_pt_regs *regs, unsigned long *aux_fp_regs)
 	interrupt_end();
 
 	while (1) {
-		if (kill_userspace_mm[0])
+		time_travel_print_bc_msg();
+
+		current_mm_sync();
+
+		/* Flush out any pending syscalls */
+		err = syscall_stub_flush(current_mm_id());
+		if (err) {
+			if (err == -ENOMEM)
+				report_enomem();
+
+			printk(UM_KERN_ERR "%s - Error flushing stub syscalls: %d",
+				__func__, -err);
 			fatal_sigsegv();
+		}
 
 		/*
 		 * This can legitimately fail if the process loads a
@@ -461,113 +473,6 @@ void userspace(struct uml_pt_regs *regs, unsigned long *aux_fp_regs)
 	}
 }
 
-static unsigned long thread_regs[MAX_REG_NR];
-static unsigned long thread_fp_regs[FP_SIZE];
-
-static int __init init_thread_regs(void)
-{
-	get_safe_registers(thread_regs, thread_fp_regs);
-	/* Set parent's instruction pointer to start of clone-stub */
-	thread_regs[REGS_IP_INDEX] = STUB_CODE +
-				(unsigned long) stub_clone_handler -
-				(unsigned long) __syscall_stub_start;
-	thread_regs[REGS_SP_INDEX] = STUB_DATA + STUB_DATA_PAGES * UM_KERN_PAGE_SIZE -
-		sizeof(void *);
-#ifdef __SIGNAL_FRAMESIZE
-	thread_regs[REGS_SP_INDEX] -= __SIGNAL_FRAMESIZE;
-#endif
-	return 0;
-}
-
-__initcall(init_thread_regs);
-
-int copy_context_skas0(unsigned long new_stack, int pid)
-{
-	int err;
-	unsigned long current_stack = current_stub_stack();
-	struct stub_data *data = (struct stub_data *) current_stack;
-	struct stub_data *child_data = (struct stub_data *) new_stack;
-	unsigned long long new_offset;
-	int new_fd = phys_mapping(uml_to_phys((void *)new_stack), &new_offset);
-
-	/*
-	 * prepare offset and fd of child's stack as argument for parent's
-	 * and child's mmap2 calls
-	 */
-	*data = ((struct stub_data) {
-		.offset	= MMAP_OFFSET(new_offset),
-		.fd     = new_fd,
-		.parent_err = -ESRCH,
-		.child_err = 0,
-	});
-
-	*child_data = ((struct stub_data) {
-		.child_err = -ESRCH,
-	});
-
-	err = ptrace_setregs(pid, thread_regs);
-	if (err < 0) {
-		err = -errno;
-		printk(UM_KERN_ERR "%s : PTRACE_SETREGS failed, pid = %d, errno = %d\n",
-		      __func__, pid, -err);
-		return err;
-	}
-
-	err = put_fp_registers(pid, thread_fp_regs);
-	if (err < 0) {
-		printk(UM_KERN_ERR "%s : put_fp_registers failed, pid = %d, err = %d\n",
-		       __func__, pid, err);
-		return err;
-	}
-
-	/*
-	 * Wait, until parent has finished its work: read child's pid from
-	 * parent's stack, and check, if bad result.
-	 */
-	err = ptrace(PTRACE_CONT, pid, 0, 0);
-	if (err) {
-		err = -errno;
-		printk(UM_KERN_ERR "Failed to continue new process, pid = %d, errno = %d\n",
-		       pid, errno);
-		return err;
-	}
-
-	wait_stub_done(pid);
-
-	pid = data->parent_err;
-	if (pid < 0) {
-		printk(UM_KERN_ERR "%s - stub-parent reports error %d\n",
-		      __func__, -pid);
-		return pid;
-	}
-
-	/*
-	 * Wait, until child has finished too: read child's result from
-	 * child's stack and check it.
-	 */
-	wait_stub_done(pid);
-	if (child_data->child_err != STUB_DATA) {
-		printk(UM_KERN_ERR "%s - stub-child %d reports error %ld\n",
-		       __func__, pid, data->child_err);
-		err = data->child_err;
-		goto out_kill;
-	}
-
-	if (ptrace(PTRACE_SETOPTIONS, pid, NULL,
-		   (void *)PTRACE_O_TRACESYSGOOD) < 0) {
-		err = -errno;
-		printk(UM_KERN_ERR "%s : PTRACE_SETOPTIONS failed, errno = %d\n",
-		       __func__, errno);
-		goto out_kill;
-	}
-
-	return pid;
-
- out_kill:
-	os_kill_ptraced_process(pid, 1);
-	return err;
-}
-
 void new_thread(void *stack, jmp_buf *buf, void (*handler)(void))
 {
 	(*buf)[0].JB_IP = (unsigned long) handler;
@@ -684,5 +589,4 @@ void reboot_skas(void)
 void __switch_mm(struct mm_id *mm_idp)
 {
 	userspace_pid[0] = mm_idp->u.pid;
-	kill_userspace_mm[0] = mm_idp->kill;
 }
diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 89ad9f4f865c..93fc82c01aba 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -17,6 +17,7 @@
 #include <sys/wait.h>
 #include <sys/time.h>
 #include <sys/resource.h>
+#include <asm/ldt.h>
 #include <asm/unistd.h>
 #include <init.h>
 #include <os.h>