[PATCH V2 1/4] KVM: x86/pmu: Do not map fixed counters >= 3 to generic perf events

Zide Chen
2026-04-23T17:54:57Z

Only fixed counters 0..2 have matching generic cross-platform
hardware perf events (INSTRUCTIONS, CPU_CYCLES, REF_CPU_CYCLES).
Therefore, perf_get_hw_event_config() is only applicable to these
counters.

KVM does not intend to emulate fixed counters >= 3 on legacy
(non-mediated) vPMU, while for mediated vPMU, KVM does not care what
the fixed counter event mappings are. Therefore, return 0 for their
eventsel.

Also remove __always_inline as BUILD_BUG_ON() is no longer needed.

Signed-off-by: Zide Chen <zide.chen@intel.com>
---
V2:
- Replace 3 in "if (index < 3)" with ARRAY_SIZE(fixed_pmc_perf_ids).
---
arch/x86/kvm/vmx/pmu_intel.c | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 27eb76e6b6a0..05a59f4acfdd 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -454,28 +454,30 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
* different perf_event is already utilizing the requested counter, but the end
* result is the same (ignoring the fact that using a general purpose counter
* will likely exacerbate counter contention).
- *
- * Forcibly inlined to allow asserting on @index at build time, and there should
- * never be more than one user.
*/
-static __always_inline u64 intel_get_fixed_pmc_eventsel(unsigned int index)
+static u64 intel_get_fixed_pmc_eventsel(unsigned int index)
{
const enum perf_hw_id fixed_pmc_perf_ids[] = {
[0] = PERF_COUNT_HW_INSTRUCTIONS,
[1] = PERF_COUNT_HW_CPU_CYCLES,
[2] = PERF_COUNT_HW_REF_CPU_CYCLES,
};
- u64 eventsel;
-
- BUILD_BUG_ON(ARRAY_SIZE(fixed_pmc_perf_ids) != KVM_MAX_NR_INTEL_FIXED_COUNTERS);
- BUILD_BUG_ON(index >= KVM_MAX_NR_INTEL_FIXED_COUNTERS);
+ u64 eventsel = 0;

/*
- * Yell if perf reports support for a fixed counter but perf doesn't
- * have a known encoding for the associated general purpose event.
+ * Fixed counters 3 and above don't have corresponding generic hardware
+ * perf event, and KVM does not intend to emulate them on non-mediated
+ * vPMU.
*/
- eventsel = perf_get_hw_event_config(fixed_pmc_perf_ids[index]);
- WARN_ON_ONCE(!eventsel && index < kvm_pmu_cap.num_counters_fixed);
+ if (index < ARRAY_SIZE(fixed_pmc_perf_ids)) {
+ /*
+ * Yell if perf reports support for a fixed counter but perf
+ * doesn't have a known encoding for the associated general
+ * purpose event.
+ */
+ eventsel = perf_get_hw_event_config(fixed_pmc_perf_ids[index]);
+ WARN_ON_ONCE(!eventsel && index < kvm_pmu_cap.num_counters_fixed);
+ }
return eventsel;
}

--
2.53.0