Drivers: hv: vmbus: Implement Direct Mode for stimer0

The 2016 version of Hyper-V offers the option to operate the guest VM
per-vcpu stimer's in Direct Mode, which means the timer interupts on its
own vector rather than queueing a VMbus message. Direct Mode reduces
timer processing overhead in both the hypervisor and the guest, and
avoids having timer interrupts pollute the VMbus interrupt stream for
the synthetic NIC and storage.  This patch enables Direct Mode by
default on stimer0 when running on a version of Hyper-V that supports
it.

In prep for coming support of Hyper-V on ARM64, the arch independent
portion of the code contains calls to routines that will be populated
on ARM64 but are not needed and do nothing on x86.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 56 insertions, 3 deletions

diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 31142b72f1b9..b1f6793acf4c 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -27,7 +27,7 @@
 #include <linux/vmalloc.h>
 #include <linux/hyperv.h>
 #include <linux/version.h>
-#include <linux/interrupt.h>
+#include <linux/random.h>
 #include <linux/clockchips.h>
 #include <asm/hyperv.h>
 #include <asm/mshyperv.h>
@@ -38,6 +38,17 @@ struct hv_context hv_context = {
 	.synic_initialized	= false,
 };
 
+/*
+ * If false, we're using the old mechanism for stimer0 interrupts
+ * where it sends a VMbus message when it expires. The old
+ * mechanism is used when running on older versions of Hyper-V
+ * that don't support Direct Mode. While Hyper-V provides
+ * four stimer's per CPU, Linux uses only stimer0.
+ */
+static bool direct_mode_enabled;
+static int stimer0_irq;
+static int stimer0_vector;
+
 #define HV_TIMER_FREQUENCY (10 * 1000 * 1000) /* 100ns period */
 #define HV_MAX_MAX_DELTA_TICKS 0xffffffff
 #define HV_MIN_DELTA_TICKS 1
@@ -53,6 +64,8 @@ int hv_init(void)
 	if (!hv_context.cpu_context)
 		return -ENOMEM;
 
+	direct_mode_enabled = ms_hyperv.misc_features &
+			HV_X64_STIMER_DIRECT_MODE_AVAILABLE;
 	return 0;
 }
 
@@ -91,6 +104,21 @@ int hv_post_message(union hv_connection_id connection_id,
 	return status & 0xFFFF;
 }
 
+/*
+ * ISR for when stimer0 is operating in Direct Mode.  Direct Mode
+ * does not use VMbus or any VMbus messages, so process here and not
+ * in the VMbus driver code.
+ */
+
+static void hv_stimer0_isr(void)
+{
+	struct hv_per_cpu_context *hv_cpu;
+
+	hv_cpu = this_cpu_ptr(hv_context.cpu_context);
+	hv_cpu->clk_evt->event_handler(hv_cpu->clk_evt);
+	add_interrupt_randomness(stimer0_vector, 0);
+}
+
 static int hv_ce_set_next_event(unsigned long delta,
 				struct clock_event_device *evt)
 {
@@ -108,6 +136,8 @@ static int hv_ce_shutdown(struct clock_event_device *evt)
 {
 	hv_init_timer(HV_X64_MSR_STIMER0_COUNT, 0);
 	hv_init_timer_config(HV_X64_MSR_STIMER0_CONFIG, 0);
+	if (direct_mode_enabled)
+		hv_disable_stimer0_percpu_irq(stimer0_irq);
 
 	return 0;
 }
@@ -116,11 +146,26 @@ static int hv_ce_set_oneshot(struct clock_event_device *evt)
 {
 	union hv_timer_config timer_cfg;
 
+	timer_cfg.as_uint64 = 0;
 	timer_cfg.enable = 1;
 	timer_cfg.auto_enable = 1;
-	timer_cfg.sintx = VMBUS_MESSAGE_SINT;
+	if (direct_mode_enabled) {
+		/*
+		 * When it expires, the timer will directly interrupt
+		 * on the specified hardware vector/IRQ.
+		 */
+		timer_cfg.direct_mode = 1;
+		timer_cfg.apic_vector = stimer0_vector;
+		hv_enable_stimer0_percpu_irq(stimer0_irq);
+	} else {
+		/*
+		 * When it expires, the timer will generate a VMbus message,
+		 * to be handled by the normal VMbus interrupt handler.
+		 */
+		timer_cfg.direct_mode = 0;
+		timer_cfg.sintx = VMBUS_MESSAGE_SINT;
+	}
 	hv_init_timer_config(HV_X64_MSR_STIMER0_CONFIG, timer_cfg.as_uint64);
-
 	return 0;
 }
 
@@ -191,6 +236,11 @@ int hv_synic_alloc(void)
 		INIT_LIST_HEAD(&hv_cpu->chan_list);
 	}
 
+	if (direct_mode_enabled &&
+	    hv_setup_stimer0_irq(&stimer0_irq, &stimer0_vector,
+				hv_stimer0_isr))
+		goto err;
+
 	return 0;
 err:
 	return -ENOMEM;
@@ -292,6 +342,9 @@ void hv_synic_clockevents_cleanup(void)
 	if (!(ms_hyperv.features & HV_X64_MSR_SYNTIMER_AVAILABLE))
 		return;
 
+	if (direct_mode_enabled)
+		hv_remove_stimer0_irq(stimer0_irq);
+
 	for_each_present_cpu(cpu) {
 		struct hv_per_cpu_context *hv_cpu
 			= per_cpu_ptr(hv_context.cpu_context, cpu);