KVM: x86/pmu: Reprogram mediated PMU event selectors on event filter updates

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]
Tested-by: Xudong Hao <xudong.hao@intel.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-26-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Dapeng Mi

arch/x86/kvm/pmu.c

@@ -520,6 +520,25 @@ 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);
@@ -528,6 +547,11 @@ 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) ||