[PATCH v5 35/44] KVM: x86/pmu: Reprogram mediated PMU event selectors on event filter updates

Sean Christopherson
2025-08-06T19:58:32Z

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

Refresh the event selectors that are programmed into hardware when a PMC
is "reprogrammed" for a mediated PMU, i.e. if userspace changes the PMU
event filters

Note, KVM doesn't utilize the reprogramming infrastructure to handle
counter overflow for mediated PMUs, as there's no need to reprogram a
non-existent perf event.

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: add a helper to document behavior, split patch and rewrite changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/pmu.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index e39ae37f0280..b4c6a7704a01 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -518,6 +518,25 @@ static bool pmc_is_event_allowed(struct kvm_pmc *pmc)
return is_fixed_event_allowed(filter, pmc->idx);
}

+static void kvm_mediated_pmu_refresh_event_filter(struct kvm_pmc *pmc)
+{
+ bool allowed = pmc_is_event_allowed(pmc);
+ struct kvm_pmu *pmu = pmc_to_pmu(pmc);
+
+ if (pmc_is_gp(pmc)) {
+ pmc->eventsel_hw &= ~ARCH_PERFMON_EVENTSEL_ENABLE;
+ if (allowed)
+ pmc->eventsel_hw |= pmc->eventsel &
+ ARCH_PERFMON_EVENTSEL_ENABLE;
+ } else {
+ u64 mask = intel_fixed_bits_by_idx(pmc->idx - KVM_FIXED_PMC_BASE_IDX, 0xf);
+
+ pmu->fixed_ctr_ctrl_hw &= ~mask;
+ if (allowed)
+ pmu->fixed_ctr_ctrl_hw |= pmu->fixed_ctr_ctrl & mask;
+ }
+}
+
static int reprogram_counter(struct kvm_pmc *pmc)
{
struct kvm_pmu *pmu = pmc_to_pmu(pmc);
@@ -526,6 +545,11 @@ static int reprogram_counter(struct kvm_pmc *pmc)
bool emulate_overflow;
u8 fixed_ctr_ctrl;

+ if (kvm_vcpu_has_mediated_pmu(pmu_to_vcpu(pmu))) {
+ kvm_mediated_pmu_refresh_event_filter(pmc);
+ return 0;
+ }
+
emulate_overflow = pmc_pause_counter(pmc);

if (!pmc_is_globally_enabled(pmc) || !pmc_is_locally_enabled(pmc) ||
--
2.50.1.565.gc32cd1483b-goog