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

[Chen, Zide]

On 11/6/2026 3:00 PM, Yosry Ahmed wrote:
> On Thu, Nov 05, 2026 at 06:39:22PM -0700, Zide Chen wrote:
>> 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)
> 
> Having to check both versions is confusing. Can they go out of sync?
> Should we clamp pmu->version to kvm_pmu_cap.version and only check the
> former?

Yes, actually, other fields (bit_width, mask_length, and
nr_{gp,fixed}_counters) in intel_pmu_refresh() are clamped to
kvm_pmu_cap as well.

pmu->version = min_t(int, eax.split.version_id, kvm_pmu_cap.version);

Without this, as more advanced PMU features are supported in KVM, we
will have to manually validate guest features, and it seems this doesn't
violate the general principles of "not policing CPUID configurations"
and using the "effective vPMU version".

> 
>> +		fixed_mask |= entry->ecx;
>>  	fixed_mask &= kvm_pmu_cap.fixed_cntr_mask64;
> 
> I see why this is needed now, to re-apply KVM's fixed counter mask after
> reading it, but if this is only needed there, it's probably clearer to
> do:
> 
> 	if (version >= 5)
> 		fixed_mask |= (entry->ecx & kvm_pmu_cap.fixed_cntr_mask64);

Agreed, this is cleaner. Will update accordingly. Thanks!