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SubjectRe: RFC: paravirtualizing perf_clock
On Sun, Oct 27, 2013 at 07:27:27PM -0600, David Ahern wrote:
> 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).
>
Yoshihiro YUNOMAE (copied) has a tool that merges guest's and host's
traces using tsc timestamp. His commit 489223edf29b adds a trace point
that reports current guest's tsc offset to support that.

> 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)
>


--
Gleb.


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