[PATCH v3 13/18] KVM: x86/pmu: Populate CPUID.0AH:ECX fixed-counter bitmap

[Zide Chen]

PerfMon v5 adds a fixed-counter support bitmap in CPUID.0AH:ECX.
This is the superset of EDX[4:0] which indicates the contiguous
counters starting from 0. For backward compatibility, it's recommended
to use the following to determine if a fixed counter is supported:

FxCtr[i]_is_supported := ECX[i] || (EDX[4:0] > i);

Originally-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
---
v3:
- Remove '&' when populating entry->ecx. (Jim)
- Gate entry->ecx on kvm_pmu_cap.version >= 5 in intel_pmu_refresh() to
  avoid enabling v5 fixed counters on hosts where KVM caps PMU at v2.
---
 arch/x86/kvm/cpuid.c         | 5 ++++-
 arch/x86/kvm/vmx/pmu_intel.c | 8 ++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 3310c370c5fa..00a2ca6d3049 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1554,7 +1554,10 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 
 		entry->eax = eax.full;
 		entry->ebx = kvm_pmu_cap.events_mask;
-		entry->ecx = 0;
+		if (kvm_pmu_cap.version >= 5)
+			entry->ecx = (u32)kvm_pmu_cap.fixed_cntr_mask64;
+		else
+			entry->ecx = 0;
 		entry->edx = edx.full;
 		break;
 	}
diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 1372091dc1f8..37b34b588601 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -597,7 +597,15 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
 				  find_first_zero_bit(kvm_pmu_cap.fixed_cntr_mask,
 						      KVM_MAX_NR_FIXED_COUNTERS));
 
+	/* FxCtr[i]_is_supported := ECX[i] || (EDX[4:0] > i) */
 	fixed_mask = BIT_ULL(nr_fixed_counters) - 1;
+	/*
+	 * Do not enable fixed counters via the v5 ECX bitmap if KVM caps
+	 * PMU version below 5 (e.g. when mediated PMU is disabled or on
+	 * older hosts), matching KVM's advertised CPUID.0AH:ECX capability.
+	 */
+	if (kvm_pmu_cap.version >= 5 && pmu->version >= 5)
+		fixed_mask |= entry->ecx;
 	fixed_mask &= kvm_pmu_cap.fixed_cntr_mask64;
 
 	/*
-- 
2.56.0