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)
