3 files changed, 53 insertions, 15 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index a565a2e70f30..562eaa938847 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -500,8 +500,23 @@ struct kvm_pmc {
 	u8 idx;
 	bool is_paused;
 	bool intr;
+	/*
+	 * Base value of the PMC counter, relative to the *consumed* count in
+	 * the associated perf_event.  This value includes counter updates from
+	 * the perf_event and emulated_count since the last time the counter
+	 * was reprogrammed, but it is *not* the current value as seen by the
+	 * guest or userspace.
+	 *
+	 * The count is relative to the associated perf_event so that KVM
+	 * doesn't need to reprogram the perf_event every time the guest writes
+	 * to the counter.
+	 */
 	u64 counter;
-	u64 prev_counter;
+	/*
+	 * PMC events triggered by KVM emulation that haven't been fully
+	 * processed, i.e. haven't undergone overflow detection.
+	 */
+	u64 emulated_counter;
 	u64 eventsel;
 	struct perf_event *perf_event;
 	struct kvm_vcpu *vcpu;
diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 3725d001239d..87cc6c8809ad 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -127,9 +127,9 @@ static void kvm_perf_overflow(struct perf_event *perf_event,
 	struct kvm_pmc *pmc = perf_event->overflow_handler_context;
 
 	/*
-	 * Ignore overflow events for counters that are scheduled to be
-	 * reprogrammed, e.g. if a PMI for the previous event races with KVM's
-	 * handling of a related guest WRMSR.
+	 * Ignore asynchronous overflow events for counters that are scheduled
+	 * to be reprogrammed, e.g. if a PMI for the previous event races with
+	 * KVM's handling of a related guest WRMSR.
 	 */
 	if (test_and_set_bit(pmc->idx, pmc_to_pmu(pmc)->reprogram_pmi))
 		return;
@@ -224,17 +224,30 @@ static int pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type, u64 config,
 	return 0;
 }
 
-static void pmc_pause_counter(struct kvm_pmc *pmc)
+static bool pmc_pause_counter(struct kvm_pmc *pmc)
 {
 	u64 counter = pmc->counter;
-
-	if (!pmc->perf_event || pmc->is_paused)
-		return;
+	u64 prev_counter;
 
 	/* update counter, reset event value to avoid redundant accumulation */
-	counter += perf_event_pause(pmc->perf_event, true);
+	if (pmc->perf_event && !pmc->is_paused)
+		counter += perf_event_pause(pmc->perf_event, true);
+
+	/*
+	 * Snapshot the previous counter *after* accumulating state from perf.
+	 * If overflow already happened, hardware (via perf) is responsible for
+	 * generating a PMI.  KVM just needs to detect overflow on emulated
+	 * counter events that haven't yet been processed.
+	 */
+	prev_counter = counter & pmc_bitmask(pmc);
+
+	counter += pmc->emulated_counter;
 	pmc->counter = counter & pmc_bitmask(pmc);
+
+	pmc->emulated_counter = 0;
 	pmc->is_paused = true;
+
+	return pmc->counter < prev_counter;
 }
 
 static bool pmc_resume_counter(struct kvm_pmc *pmc)
@@ -289,6 +302,15 @@ static void pmc_update_sample_period(struct kvm_pmc *pmc)
 
 void pmc_write_counter(struct kvm_pmc *pmc, u64 val)
 {
+	/*
+	 * Drop any unconsumed accumulated counts, the WRMSR is a write, not a
+	 * read-modify-write.  Adjust the counter value so that its value is
+	 * relative to the current count, as reading the current count from
+	 * perf is faster than pausing and repgrogramming the event in order to
+	 * reset it to '0'.  Note, this very sneakily offsets the accumulated
+	 * emulated count too, by using pmc_read_counter()!
+	 */
+	pmc->emulated_counter = 0;
 	pmc->counter += val - pmc_read_counter(pmc);
 	pmc->counter &= pmc_bitmask(pmc);
 	pmc_update_sample_period(pmc);
@@ -428,14 +450,15 @@ static void reprogram_counter(struct kvm_pmc *pmc)
 	struct kvm_pmu *pmu = pmc_to_pmu(pmc);
 	u64 eventsel = pmc->eventsel;
 	u64 new_config = eventsel;
+	bool emulate_overflow;
 	u8 fixed_ctr_ctrl;
 
-	pmc_pause_counter(pmc);
+	emulate_overflow = pmc_pause_counter(pmc);
 
 	if (!pmc_event_is_allowed(pmc))
 		goto reprogram_complete;
 
-	if (pmc->counter < pmc->prev_counter)
+	if (emulate_overflow)
 		__kvm_perf_overflow(pmc, false);
 
 	if (eventsel & ARCH_PERFMON_EVENTSEL_PIN_CONTROL)
@@ -475,7 +498,6 @@ static void reprogram_counter(struct kvm_pmc *pmc)
 
 reprogram_complete:
 	clear_bit(pmc->idx, (unsigned long *)&pmc_to_pmu(pmc)->reprogram_pmi);
-	pmc->prev_counter = 0;
 }
 
 void kvm_pmu_handle_event(struct kvm_vcpu *vcpu)
@@ -701,6 +723,7 @@ static void kvm_pmu_reset(struct kvm_vcpu *vcpu)
 
 		pmc_stop_counter(pmc);
 		pmc->counter = 0;
+		pmc->emulated_counter = 0;
 
 		if (pmc_is_gp(pmc))
 			pmc->eventsel = 0;
@@ -772,8 +795,7 @@ void kvm_pmu_destroy(struct kvm_vcpu *vcpu)
 
 static void kvm_pmu_incr_counter(struct kvm_pmc *pmc)
 {
-	pmc->prev_counter = pmc->counter;
-	pmc->counter = (pmc->counter + 1) & pmc_bitmask(pmc);
+	pmc->emulated_counter++;
 	kvm_pmu_request_counter_reprogram(pmc);
 }
 
diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
index cae85e550f60..7caeb3d8d4fd 100644
--- a/arch/x86/kvm/pmu.h
+++ b/arch/x86/kvm/pmu.h
@@ -66,7 +66,8 @@ static inline u64 pmc_read_counter(struct kvm_pmc *pmc)
 {
 	u64 counter, enabled, running;
 
-	counter = pmc->counter;
+	counter = pmc->counter + pmc->emulated_counter;
+
 	if (pmc->perf_event && !pmc->is_paused)
 		counter += perf_event_read_value(pmc->perf_event,
 						 &enabled, &running);