[PATCH v6 23/44] KVM: x86/pmu: Bypass perf checks when emulating mediated PMU counter accesses

Sean Christopherson
2025-12-06T00:18:12Z

From: Dapeng Mi <dapeng1.mi@linux.intel.com>

When emulating a PMC counter read or write for a mediated PMU, bypass the
perf checks and emulated_counter logic as the counters aren't proxied
through perf, i.e. pmc->counter always holds the guest's up-to-date value,
and thus there's no need to defer emulated overflow checks.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Co-developed-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
[sean: split from event filtering change, write shortlog+changelog]
Reviewed-by: Sandipan Das <sandipan.das@amd.com>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/pmu.c | 5 +++++
arch/x86/kvm/pmu.h | 3 +++
2 files changed, 8 insertions(+)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 57833f29a746..621722e8cc7e 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -381,6 +381,11 @@ static void pmc_update_sample_period(struct kvm_pmc *pmc)

void pmc_write_counter(struct kvm_pmc *pmc, u64 val)
{
+ if (kvm_vcpu_has_mediated_pmu(pmc->vcpu)) {
+ pmc->counter = val & pmc_bitmask(pmc);
+ return;
+ }
+
/*
* Drop any unconsumed accumulated counts, the WRMSR is a write, not a
* read-modify-write. Adjust the counter value so that its value is
diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
index 356b08e92bc9..9a199109d672 100644
--- a/arch/x86/kvm/pmu.h
+++ b/arch/x86/kvm/pmu.h
@@ -111,6 +111,9 @@ static inline u64 pmc_read_counter(struct kvm_pmc *pmc)
{
u64 counter, enabled, running;

+ if (kvm_vcpu_has_mediated_pmu(pmc->vcpu))
+ return pmc->counter & pmc_bitmask(pmc);
+
counter = pmc->counter + pmc->emulated_counter;

if (pmc->perf_event && !pmc->is_paused)
--
2.52.0.223.gf5cc29aaa4-goog