Re: [PATCH v2] KVM: x86/pmu: Reduce counter period change overhead and delay the effective time

[Like Xu <like.xu@xxxxxxxxxxxxxxx>]

Hi Paolo,

Could you please take a look at this patch?
If there is anything needs to be improved, please let me know.

Thanks,
Like Xu

On 2020/3/26 20:47, Like Xu wrote:
> Anyone to help review this change?
>
> Thanks,
> Like Xu
>
> On 2020/3/17 16:14, Like Xu wrote:
> > The cost of perf_event_period() is unstable, and when the guest samples
> > multiple events, the overhead increases dramatically (5378 ns on E5-2699).
> >
> > For a non-running counter, the effective time of the new period is when
> > its corresponding enable bit is enabled. Calling perf_event_period()
> > in advance is superfluous. For a running counter, it's safe to delay the
> > effective time until the KVM_REQ_PMU event is handled. If there are
> > multiple perf_event_period() calls before handling KVM_REQ_PMU,
> > it helps to reduce the total cost.
> >
> > Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/pmu.c           | 11 -----------
> >   arch/x86/kvm/pmu.h           | 11 +++++++++++
> >   arch/x86/kvm/vmx/pmu_intel.c | 10 ++++------
> >   3 files changed, 15 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index d1f8ca57d354..527a8bb85080 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -437,17 +437,6 @@ void kvm_pmu_init(struct kvm_vcpu *vcpu)
> >       kvm_pmu_refresh(vcpu);
> >   }
> > -static inline bool pmc_speculative_in_use(struct kvm_pmc *pmc)
> > -{
> > -    struct kvm_pmu *pmu = pmc_to_pmu(pmc);
> > -
> > -    if (pmc_is_fixed(pmc))
> > -        return fixed_ctrl_field(pmu->fixed_ctr_ctrl,
> > -            pmc->idx - INTEL_PMC_IDX_FIXED) & 0x3;
> > -
> > -    return pmc->eventsel & ARCH_PERFMON_EVENTSEL_ENABLE;
> > -}
> > -
> >   /* Release perf_events for vPMCs that have been unused for a full time 
> > slice.  */
> >   void kvm_pmu_cleanup(struct kvm_vcpu *vcpu)
> >   {
> > diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> > index d7da2b9e0755..cd112e825d2c 100644
> > --- a/arch/x86/kvm/pmu.h
> > +++ b/arch/x86/kvm/pmu.h
> > @@ -138,6 +138,17 @@ static inline u64 get_sample_period(struct kvm_pmc 
> > *pmc, u64 counter_value)
> >       return sample_period;
> >   }
> > +static inline bool pmc_speculative_in_use(struct kvm_pmc *pmc)
> > +{
> > +    struct kvm_pmu *pmu = pmc_to_pmu(pmc);
> > +
> > +    if (pmc_is_fixed(pmc))
> > +        return fixed_ctrl_field(pmu->fixed_ctr_ctrl,
> > +            pmc->idx - INTEL_PMC_IDX_FIXED) & 0x3;
> > +
> > +    return pmc->eventsel & ARCH_PERFMON_EVENTSEL_ENABLE;
> > +}
> > +
> >   void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel);
> >   void reprogram_fixed_counter(struct kvm_pmc *pmc, u8 ctrl, int fixed_idx);
> >   void reprogram_counter(struct kvm_pmu *pmu, int pmc_idx);
> > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > index 7c857737b438..20f654a0c09b 100644
> > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > @@ -263,15 +263,13 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, 
> > struct msr_data *msr_info)
> >               if (!msr_info->host_initiated)
> >                   data = (s64)(s32)data;
> >               pmc->counter += data - pmc_read_counter(pmc);
> > -            if (pmc->perf_event)
> > -                perf_event_period(pmc->perf_event,
> > -                          get_sample_period(pmc, data));
> > +            if (pmc_speculative_in_use(pmc))
> > +                kvm_make_request(KVM_REQ_PMU, vcpu);
> >               return 0;
> >           } else if ((pmc = get_fixed_pmc(pmu, msr))) {
> >               pmc->counter += data - pmc_read_counter(pmc);
> > -            if (pmc->perf_event)
> > -                perf_event_period(pmc->perf_event,
> > -                          get_sample_period(pmc, data));
> > +            if (pmc_speculative_in_use(pmc))
> > +                kvm_make_request(KVM_REQ_PMU, vcpu);
> >               return 0;
> >           } else if ((pmc = get_gp_pmc(pmu, msr, MSR_P6_EVNTSEL0))) {
> >               if (data == pmc->eventsel)