s390/mm: rework memcpy_real() to avoid DAT-off mode

Function memcpy_real() is an univeral data mover that does not
require DAT mode to be able reading from a physical address.
Its advantage is an ability to read from any address, even
those for which no kernel virtual mapping exists.

Although memcpy_real() is interrupt-safe, there are no handlers
that make use of this function. The compiler instrumentation
have to be disabled and separate no-DAT stack used to allow
execution of the function once DAT mode is disabled.

Rework memcpy_real() to overcome these shortcomings. As result,
data copying (which is primarily reading out a crashed system
memory by a user process) is executed on a regular stack with
enabled interrupts. Also, use of memcpy_real_buf swap buffer
becomes unnecessary and the swapping is eliminated.

The above is achieved by using a fixed virtual address range
that spans a single page and remaps that page repeatedly when
memcpy_real() is called for a particular physical address.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

2 files changed, 4 insertions, 24 deletions

diff --git a/arch/s390/kernel/crash_dump.c b/arch/s390/kernel/crash_dump.c
index bad8f47fc5d6..438fe696a4a3 100644
--- a/arch/s390/kernel/crash_dump.c
+++ b/arch/s390/kernel/crash_dump.c
@@ -21,6 +21,7 @@
 #include <asm/elf.h>
 #include <asm/ipl.h>
 #include <asm/sclp.h>
+#include <asm/maccess.h>
 
 #define PTR_ADD(x, y) (((char *) (x)) + ((unsigned long) (y)))
 #define PTR_SUB(x, y) (((char *) (x)) - ((unsigned long) (y)))
@@ -53,8 +54,6 @@ struct save_area {
 };
 
 static LIST_HEAD(dump_save_areas);
-static DEFINE_MUTEX(memcpy_real_mutex);
-static char memcpy_real_buf[PAGE_SIZE];
 
 /*
  * Allocate a save area
@@ -116,26 +115,6 @@ void __init save_area_add_vxrs(struct save_area *sa, __vector128 *vxrs)
 	memcpy(sa->vxrs_high, vxrs + 16, 16 * sizeof(__vector128));
 }
 
-static size_t copy_to_iter_real(struct iov_iter *iter, unsigned long src, size_t count)
-{
-	size_t len, copied, res = 0;
-
-	mutex_lock(&memcpy_real_mutex);
-	while (count) {
-		len = min(PAGE_SIZE, count);
-		if (memcpy_real(memcpy_real_buf, src, len))
-			break;
-		copied = copy_to_iter(memcpy_real_buf, len, iter);
-		count -= copied;
-		src += copied;
-		res += copied;
-		if (copied < len)
-			break;
-	}
-	mutex_unlock(&memcpy_real_mutex);
-	return res;
-}
-
 size_t copy_oldmem_iter(struct iov_iter *iter, unsigned long src, size_t count)
 {
 	size_t len, copied, res = 0;
@@ -156,7 +135,7 @@ size_t copy_oldmem_iter(struct iov_iter *iter, unsigned long src, size_t count)
 			} else {
 				len = count;
 			}
-			copied = copy_to_iter_real(iter, src, len);
+			copied = memcpy_real_iter(iter, src, len);
 		}
 		count -= copied;
 		src += copied;
diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
index d1390899a8e7..ab19ddb09d65 100644
--- a/arch/s390/kernel/setup.c
+++ b/arch/s390/kernel/setup.c
@@ -74,6 +74,7 @@
 #include <asm/alternative.h>
 #include <asm/nospec-branch.h>
 #include <asm/mem_detect.h>
+#include <asm/maccess.h>
 #include <asm/uv.h>
 #include <asm/asm-offsets.h>
 #include "entry.h"
@@ -1050,7 +1051,7 @@ void __init setup_arch(char **cmdline_p)
 	 * Create kernel page tables and switch to virtual addressing.
 	 */
         paging_init();
-
+	memcpy_real_init();
 	/*
 	 * After paging_init created the kernel page table, the new PSWs
 	 * in lowcore can now run with DAT enabled.