KVM: x86/pmu: Calculate set of to-be-emulated PMCs at time of WRMSRs

Calculate and track PMCs that are counting instructions/branches retired
when the PMC's event selector (or fixed counter control) is modified
instead evaluating the event selector on-demand.  Immediately recalc a
PMC's configuration on writes to avoid false negatives/positives when
KVM skips an emulated WRMSR, which is guaranteed to occur before the
main run loop processes KVM_REQ_PMU.

Out of an abundance of caution, and because it's relatively cheap, recalc
reprogrammed PMCs in kvm_pmu_handle_event() as well.  Recalculating in
response to KVM_REQ_PMU _should_ be unnecessary, but for now be paranoid
to avoid introducing easily-avoidable bugs in edge cases.  The code can be
removed in the future if necessary, e.g. in the unlikely event that the
overhead of recalculating to-be-emulated PMCs is noticeable.

Note!  Deliberately don't check the PMU event filters, as doing so could
result in KVM consuming stale information.

Tracking which PMCs are counting branches/instructions will allow grabbing
SRCU in the fastpath VM-Exit handlers if and only if a PMC event might be
triggered (to consult the event filters), and will also allow the upcoming
mediated PMU to do the right thing with respect to counting instructions
(the mediated PMU won't be able to update PMCs in the VM-Exit fastpath).

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20250805190526.1453366-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/include/asm/kvm_host.h

@@ -579,6 +579,9 @@ struct kvm_pmu {
 	DECLARE_BITMAP(all_valid_pmc_idx, X86_PMC_IDX_MAX);
 	DECLARE_BITMAP(pmc_in_use, X86_PMC_IDX_MAX);
 
+	DECLARE_BITMAP(pmc_counting_instructions, X86_PMC_IDX_MAX);
+	DECLARE_BITMAP(pmc_counting_branches, X86_PMC_IDX_MAX);
+
 	u64 ds_area;
 	u64 pebs_enable;
 	u64 pebs_enable_rsvd;

arch/x86/kvm/pmu.c

@@ -542,6 +542,47 @@ static int reprogram_counter(struct kvm_pmc *pmc)
 				     eventsel & ARCH_PERFMON_EVENTSEL_INT);
 }
 
+static bool pmc_is_event_match(struct kvm_pmc *pmc, u64 eventsel)
+{
+	/*
+	 * Ignore checks for edge detect (all events currently emulated by KVM
+	 * are always rising edges), pin control (unsupported by modern CPUs),
+	 * and counter mask and its invert flag (KVM doesn't emulate multiple
+	 * events in a single clock cycle).
+	 *
+	 * Note, the uppermost nibble of AMD's mask overlaps Intel's IN_TX (bit
+	 * 32) and IN_TXCP (bit 33), as well as two reserved bits (bits 35:34).
+	 * Checking the "in HLE/RTM transaction" flags is correct as the vCPU
+	 * can't be in a transaction if KVM is emulating an instruction.
+	 *
+	 * Checking the reserved bits might be wrong if they are defined in the
+	 * future, but so could ignoring them, so do the simple thing for now.
+	 */
+	return !((pmc->eventsel ^ eventsel) & AMD64_RAW_EVENT_MASK_NB);
+}
+
+void kvm_pmu_recalc_pmc_emulation(struct kvm_pmu *pmu, struct kvm_pmc *pmc)
+{
+	bitmap_clear(pmu->pmc_counting_instructions, pmc->idx, 1);
+	bitmap_clear(pmu->pmc_counting_branches, pmc->idx, 1);
+
+	/*
+	 * Do NOT consult the PMU event filters, as the filters must be checked
+	 * at the time of emulation to ensure KVM uses fresh information, e.g.
+	 * omitting a PMC from a bitmap could result in a missed event if the
+	 * filter is changed to allow counting the event.
+	 */
+	if (!pmc_speculative_in_use(pmc))
+		return;
+
+	if (pmc_is_event_match(pmc, kvm_pmu_eventsel.INSTRUCTIONS_RETIRED))
+		bitmap_set(pmu->pmc_counting_instructions, pmc->idx, 1);
+
+	if (pmc_is_event_match(pmc, kvm_pmu_eventsel.BRANCH_INSTRUCTIONS_RETIRED))
+		bitmap_set(pmu->pmc_counting_branches, pmc->idx, 1);
+}
+EXPORT_SYMBOL_GPL(kvm_pmu_recalc_pmc_emulation);
+
 void kvm_pmu_handle_event(struct kvm_vcpu *vcpu)
 {
 	DECLARE_BITMAP(bitmap, X86_PMC_IDX_MAX);
@@ -577,6 +618,9 @@ void kvm_pmu_handle_event(struct kvm_vcpu *vcpu)
 	 */
 	if (unlikely(pmu->need_cleanup))
 		kvm_pmu_cleanup(vcpu);
+
+	kvm_for_each_pmc(pmu, pmc, bit, bitmap)
+		kvm_pmu_recalc_pmc_emulation(pmu, pmc);
 }
 
 int kvm_pmu_check_rdpmc_early(struct kvm_vcpu *vcpu, unsigned int idx)
@@ -915,7 +959,8 @@ static inline bool cpl_is_matched(struct kvm_pmc *pmc)
 							 select_user;
 }
 
-static void kvm_pmu_trigger_event(struct kvm_vcpu *vcpu, u64 eventsel)
+static void kvm_pmu_trigger_event(struct kvm_vcpu *vcpu,
+				  const unsigned long *event_pmcs)
 {
 	DECLARE_BITMAP(bitmap, X86_PMC_IDX_MAX);
 	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
@@ -924,29 +969,17 @@ static void kvm_pmu_trigger_event(struct kvm_vcpu *vcpu, u64 eventsel)
 
 	BUILD_BUG_ON(sizeof(pmu->global_ctrl) * BITS_PER_BYTE != X86_PMC_IDX_MAX);
 
+	if (bitmap_empty(event_pmcs, X86_PMC_IDX_MAX))
+		return;
+
 	if (!kvm_pmu_has_perf_global_ctrl(pmu))
-		bitmap_copy(bitmap, pmu->all_valid_pmc_idx, X86_PMC_IDX_MAX);
-	else if (!bitmap_and(bitmap, pmu->all_valid_pmc_idx,
+		bitmap_copy(bitmap, event_pmcs, X86_PMC_IDX_MAX);
+	else if (!bitmap_and(bitmap, event_pmcs,
 			     (unsigned long *)&pmu->global_ctrl, X86_PMC_IDX_MAX))
 		return;
 
 	kvm_for_each_pmc(pmu, pmc, i, bitmap) {
-		/*
-		 * Ignore checks for edge detect (all events currently emulated
-		 * but KVM are always rising edges), pin control (unsupported
-		 * by modern CPUs), and counter mask and its invert flag (KVM
-		 * doesn't emulate multiple events in a single clock cycle).
-		 *
-		 * Note, the uppermost nibble of AMD's mask overlaps Intel's
-		 * IN_TX (bit 32) and IN_TXCP (bit 33), as well as two reserved
-		 * bits (bits 35:34).  Checking the "in HLE/RTM transaction"
-		 * flags is correct as the vCPU can't be in a transaction if
-		 * KVM is emulating an instruction.  Checking the reserved bits
-		 * might be wrong if they are defined in the future, but so
-		 * could ignoring them, so do the simple thing for now.
-		 */
-		if (((pmc->eventsel ^ eventsel) & AMD64_RAW_EVENT_MASK_NB) ||
-		    !pmc_event_is_allowed(pmc) || !cpl_is_matched(pmc))
+		if (!pmc_event_is_allowed(pmc) || !cpl_is_matched(pmc))
 			continue;
 
 		kvm_pmu_incr_counter(pmc);
@@ -955,13 +988,13 @@ static void kvm_pmu_trigger_event(struct kvm_vcpu *vcpu, u64 eventsel)
 
 void kvm_pmu_instruction_retired(struct kvm_vcpu *vcpu)
 {
-	kvm_pmu_trigger_event(vcpu, kvm_pmu_eventsel.INSTRUCTIONS_RETIRED);
+	kvm_pmu_trigger_event(vcpu, vcpu_to_pmu(vcpu)->pmc_counting_instructions);
 }
 EXPORT_SYMBOL_GPL(kvm_pmu_instruction_retired);
 
 void kvm_pmu_branch_retired(struct kvm_vcpu *vcpu)
 {
-	kvm_pmu_trigger_event(vcpu, kvm_pmu_eventsel.BRANCH_INSTRUCTIONS_RETIRED);
+	kvm_pmu_trigger_event(vcpu, vcpu_to_pmu(vcpu)->pmc_counting_branches);
 }
 EXPORT_SYMBOL_GPL(kvm_pmu_branch_retired);
 

arch/x86/kvm/pmu.h

@@ -176,8 +176,12 @@ extern struct x86_pmu_capability kvm_pmu_cap;
 
 void kvm_init_pmu_capability(const struct kvm_pmu_ops *pmu_ops);
 
+void kvm_pmu_recalc_pmc_emulation(struct kvm_pmu *pmu, struct kvm_pmc *pmc);
+
 static inline void kvm_pmu_request_counter_reprogram(struct kvm_pmc *pmc)
 {
+	kvm_pmu_recalc_pmc_emulation(pmc_to_pmu(pmc), pmc);
+
 	set_bit(pmc->idx, pmc_to_pmu(pmc)->reprogram_pmi);
 	kvm_make_request(KVM_REQ_PMU, pmc->vcpu);
 }