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

[Like Xu <like.xu@xxxxxxxxxxxxxxx>]

Ping.

On 2020/4/8 22:04, Like Xu wrote:
> 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)