KVM: x86: Adjust counter sample period after a wrmsr

The sample_period of a counter tracks when that counter will
overflow and set global status/trigger a PMI. However this currently
only gets set when the initial counter is created or when a counter is
resumed; this updates the sample period after a wrmsr so running
counters will accurately reflect their new value.
Signed-off-by: NEric Hankland <ehankland@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/pmu.c

@@ -111,7 +111,7 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
 		.config = config,
 	};
 
-	attr.sample_period = (-pmc->counter) & pmc_bitmask(pmc);
+	attr.sample_period = get_sample_period(pmc, pmc->counter);
 
 	if (in_tx)
 		attr.config |= HSW_IN_TX;
@@ -158,7 +158,7 @@ static bool pmc_resume_counter(struct kvm_pmc *pmc)
 
 	/* recalibrate sample period and check if it's accepted by perf core */
 	if (perf_event_period(pmc->perf_event,
-			(-pmc->counter) & pmc_bitmask(pmc)))
+			      get_sample_period(pmc, pmc->counter)))
 		return false;
 
 	/* reuse perf_event to serve as pmc_reprogram_counter() does*/

arch/x86/kvm/pmu.h

@@ -129,6 +129,15 @@ static inline struct kvm_pmc *get_fixed_pmc(struct kvm_pmu *pmu, u32 msr)
 	return NULL;
 }
 
+static inline u64 get_sample_period(struct kvm_pmc *pmc, u64 counter_value)
+{
+	u64 sample_period = (-counter_value) & pmc_bitmask(pmc);
+
+	if (!sample_period)
+		sample_period = pmc_bitmask(pmc) + 1;
+	return sample_period;
+}
+
 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);

arch/x86/kvm/vmx/pmu_intel.c

@@ -263,9 +263,15 @@ 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));
 			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));
 			return 0;
 		} else if ((pmc = get_gp_pmc(pmu, msr, MSR_P6_EVNTSEL0))) {
 			if (data == pmc->eventsel)