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

[Like Xu]

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)
-- 
2.21.1