[PATCH v5 38/44] KVM: x86/pmu: Disallow emulation in the fastpath if mediated PMCs are active

Sean Christopherson
2025-08-06T19:58:38Z

Don't handle exits in the fastpath if emulation is required, i.e. if an
instruction needs to be skipped, the mediated PMU is enabled, and one or
more PMCs is counting instructions. With the mediated PMU, KVM's cache of
PMU state is inconsistent with respect to hardware until KVM exits the
inner run loop (when the mediated PMU is "put").

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/pmu.h | 10 ++++++++++
arch/x86/kvm/x86.c | 9 +++++++++
2 files changed, 19 insertions(+)

diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
index e2e2d8476a3f..a0cd42cbea9d 100644
--- a/arch/x86/kvm/pmu.h
+++ b/arch/x86/kvm/pmu.h
@@ -234,6 +234,16 @@ static inline bool pmc_is_globally_enabled(struct kvm_pmc *pmc)
return test_bit(pmc->idx, (unsigned long *)&pmu->global_ctrl);
}

+static inline bool kvm_pmu_is_fastpath_emulation_allowed(struct kvm_vcpu *vcpu)
+{
+ struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
+
+ return !kvm_vcpu_has_mediated_pmu(vcpu) ||
+ !bitmap_intersects(pmu->pmc_counting_instructions,
+ (unsigned long *)&pmu->global_ctrl,
+ X86_PMC_IDX_MAX);
+}
+
void kvm_pmu_deliver_pmi(struct kvm_vcpu *vcpu);
void kvm_pmu_handle_event(struct kvm_vcpu *vcpu);
int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned pmc, u64 *data);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7fb94ef64e18..6bdf7ef0b535 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2092,6 +2092,9 @@ EXPORT_SYMBOL_GPL(kvm_emulate_invd);

fastpath_t handle_fastpath_invd(struct kvm_vcpu *vcpu)
{
+ if (!kvm_pmu_is_fastpath_emulation_allowed(vcpu))
+ return EXIT_FASTPATH_NONE;
+
if (!kvm_emulate_invd(vcpu))
return EXIT_FASTPATH_EXIT_USERSPACE;

@@ -2151,6 +2154,9 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
u64 data = kvm_read_edx_eax(vcpu);
u32 msr = kvm_rcx_read(vcpu);

+ if (!kvm_pmu_is_fastpath_emulation_allowed(vcpu))
+ return EXIT_FASTPATH_NONE;
+
switch (msr) {
case APIC_BASE_MSR + (APIC_ICR >> 4):
if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(vcpu->arch.apic) ||
@@ -11267,6 +11273,9 @@ EXPORT_SYMBOL_GPL(kvm_emulate_halt);

fastpath_t handle_fastpath_hlt(struct kvm_vcpu *vcpu)
{
+ if (!kvm_pmu_is_fastpath_emulation_allowed(vcpu))
+ return EXIT_FASTPATH_NONE;
+
if (!kvm_emulate_halt(vcpu))
return EXIT_FASTPATH_EXIT_USERSPACE;

--
2.50.1.565.gc32cd1483b-goog