RFC: paravirtualizing perf_clock

[David Ahern]

Often when debugging performance problems in a virtualized environment 
you need to correlate what is happening in the guest with what is 
happening in the host. To correlate events you need a common time basis 
(or the ability to directly correlate the two).

The attached patch paravirtualizes perf_clock, pulling the timestamps in 
VMs from the host using an MSR read if the option is available (exposed 
via KVM feature flag). I realize this is not the correct end code but it 
illustrates what I would like to see -- host and guests using the same 
perf_clock so timestamps directly correlate.

Any suggestions on how to do this and without impacting performance. I 
noticed the MSR path seems to take about twice as long as the current 
implementation (which I believe results in rdtsc in the VM for x86 with 
stable TSC).

David


diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 94dc8ca434e0..5a023ddf085e 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -24,6 +24,7 @@
 #define KVM_FEATURE_STEAL_TIME		5
 #define KVM_FEATURE_PV_EOI		6
 #define KVM_FEATURE_PV_UNHALT		7
+#define KVM_FEATURE_PV_PERF_CLOCK	8
 
 /* The last 8 bits are used to indicate how to interpret the flags field
  * in pvclock structure. If no bits are set, all flags are ignored.
@@ -40,6 +41,7 @@
 #define MSR_KVM_ASYNC_PF_EN 0x4b564d02
 #define MSR_KVM_STEAL_TIME  0x4b564d03
 #define MSR_KVM_PV_EOI_EN      0x4b564d04
+#define MSR_KVM_PV_PERF_CLOCK      0x4b564d05
 
 struct kvm_steal_time {
 	__u64 steal;
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 9d8449158cf9..fb7824a64823 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -25,6 +25,7 @@
 #include <linux/cpu.h>
 #include <linux/bitops.h>
 #include <linux/device.h>
+#include <linux/kvm_para.h>
 
 #include <asm/apic.h>
 #include <asm/stacktrace.h>
@@ -34,6 +35,7 @@
 #include <asm/timer.h>
 #include <asm/desc.h>
 #include <asm/ldt.h>
+#include <asm/kvm_para.h>
 
 #include "perf_event.h"
 
@@ -52,6 +54,38 @@ u64 __read_mostly hw_cache_extra_regs
 				[PERF_COUNT_HW_CACHE_OP_MAX]
 				[PERF_COUNT_HW_CACHE_RESULT_MAX];
 
+
+#ifdef CONFIG_PARAVIRT
+
+static int have_pv_perf_clock;
+
+static void __init perf_clock_init(void)
+{
+	if (kvm_para_available() &&
+		kvm_para_has_feature(KVM_FEATURE_PV_PERF_CLOCK)) {
+		have_pv_perf_clock = 1;
+	}
+}
+
+u64 perf_clock(void)
+{
+	if (have_pv_perf_clock)
+		return native_read_msr(MSR_KVM_PV_PERF_CLOCK);
+
+	/* otherwise return local_clock */
+	return local_clock();
+}
+
+#else
+u64 perf_clock(void)
+{
+	return local_clock();
+}
+
+static inline void __init perf_clock_init(void)
+{
+}
+#endif
 /*
  * Propagate event elapsed time into the generic event.
  * Can only be executed on the CPU where the event is active.
@@ -1496,6 +1530,8 @@ static int __init init_hw_perf_events(void)
 	struct x86_pmu_quirk *quirk;
 	int err;
 
+	perf_clock_init();
+
 	pr_info("Performance Events: ");
 
 	switch (boot_cpu_data.x86_vendor) {
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index b110fe6c03d4..5b258a18f9c0 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -414,7 +414,8 @@ static int do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 			     (1 << KVM_FEATURE_ASYNC_PF) |
 			     (1 << KVM_FEATURE_PV_EOI) |
 			     (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT) |
-			     (1 << KVM_FEATURE_PV_UNHALT);
+			     (1 << KVM_FEATURE_PV_UNHALT) |
+			     (1 << KVM_FEATURE_PV_PERF_CLOCK);
 
 		if (sched_info_on())
 			entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e5ca72a5cdb6..61ec1f1c7d38 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2418,6 +2418,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
 	case MSR_KVM_PV_EOI_EN:
 		data = vcpu->arch.pv_eoi.msr_val;
 		break;
+	case MSR_KVM_PV_PERF_CLOCK:
+		data = perf_clock();
+		break;
 	case MSR_IA32_P5_MC_ADDR:
 	case MSR_IA32_P5_MC_TYPE:
 	case MSR_IA32_MCG_CAP:
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index c8ba627c1d60..c8a51954ea9e 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -865,4 +865,7 @@ _name##_show(struct device *dev,					\
 									\
 static struct device_attribute format_attr_##_name = __ATTR_RO(_name)
 
+#if defined(CONFIG_X86_64)
+u64 perf_clock(void);
+#endif
 #endif /* _LINUX_PERF_EVENT_H */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index d49a9d29334c..b073975af05a 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -290,10 +290,18 @@ extern __weak const char *perf_pmu_name(void)
 	return "pmu";
 }
 
+#if defined(CONFIG_X86_64)
+__weak u64 perf_clock(void)
+{
+	return local_clock();
+}
+EXPORT_SYMBOL(perf_clock);
+#else
 static inline u64 perf_clock(void)
 {
 	return local_clock();
 }
+#endif
 
 static inline struct perf_cpu_context *
 __get_cpu_context(struct perf_event_context *ctx)
-- 
1.7.12.4 (Apple Git-37)