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    Subject[PATCH v2] KVM: x86/pmu: Reduce counter period change overhead and delay the effective time
    Date
    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@linux.intel.com>
    ---
    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)
    --
    2.21.1
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