Re: [PATCH v3 08/18] KVM: x86/pmu: Switch to bitmask-based KVM PMU capabilities

[Chen, Zide]

On 11/6/2026 2:05 PM, Yosry Ahmed wrote:

>> @@ -1903,7 +1911,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>>  
>>  		cpuid_entry_override(entry, CPUID_8000_0022_EAX);
>>  
>> -		ebx.split.num_core_pmc = kvm_pmu_cap.num_counters_gp;
>> +		ebx.split.num_core_pmc = hweight64(kvm_pmu_cap.cntr_mask64);
> 
> Should this be the number of contiguous counters?

Yes, to match the Intel code, and to be defensive, I may update it to this:

--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1903,7 +1911,10 @@ static inline int __do_cpuid_func(struct
kvm_cpuid_array *array, u32 function)

                cpuid_entry_override(entry, CPUID_8000_0022_EAX);

-               ebx.split.num_core_pmc = hweight64(kvm_pmu_cap.cntr_mask64);
+               /* Contiguous GP counters only. */
+               ebx.split.num_core_pmc =
+                       find_first_zero_bit(kvm_pmu_cap.cntr_mask,
+                                           KVM_MAX_NR_GP_COUNTERS);

And replace other places of hweight64(kvm_pmu_cap.cntr_mask64) to
find_first_zero_bit() in AMD code.

> 
>>  		entry->ebx = ebx.full;
>>  		break;
>>  	}
> [..]  
>> @@ -516,7 +517,7 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>>  	union cpuid10_edx edx;
>>  	u64 perf_capabilities;
>>  	u64 fixed_mask;
>> -	int nr_gp_counters;
>> +	int nr_gp_counters, nr_fixed_counters;
>>  	u64 counter_rsvd;
>>  
>>  	if (!lbr_desc)
>> @@ -550,16 +551,25 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>>  				      kvm_pmu_cap.events_mask_len);
>>  	pmu->available_event_types = ~entry->ebx & (BIT_ULL(eax.split.mask_length) - 1);
>>  
>> -	fixed_mask = BIT_ULL(edx.split.num_counters_fixed) - 1;
>> -	fixed_mask &= BIT_ULL(kvm_pmu_cap.num_counters_fixed) - 1;
>> +	/*
>> +	 * CPUID.0AH:EDX[4:0] can only enumerate contiguous counters. Clamp the
>> +	 * value to the number of contiguous fixed counters supported by KVM.
>> +	 */
>> +	nr_fixed_counters = min_t(int, edx.split.num_counters_fixed,
>> +				  find_first_zero_bit(kvm_pmu_cap.fixed_cntr_mask,
>> +						      KVM_MAX_NR_FIXED_COUNTERS));
>> +
>> +	fixed_mask = BIT_ULL(nr_fixed_counters) - 1;
>> +	fixed_mask &= kvm_pmu_cap.fixed_cntr_mask64;
> 
> Isn't nr_fixed_counters already clamped to the number of contiguous bits
> in fixed_cntr_mask?

Yes, you are right.
I will drop this redundant '&=' statement from this patch:

> 
>>  
>>  	/*
>> -	 * The number of counters comes from guest CPUID data. Clamp the value
>> -	 * to avoid a shift-by-64 in BIT_ULL().
>> +	 * CPUID.0AH:EAX[15:8] can only enumerate contiguous counters. Clamp the
>> +	 * value to the number of contiguous GP counters supported by KVM.
>>  	 */
>> -	nr_gp_counters = min_t(int, eax.split.num_counters, X86_PMC_IDX_MAX - 1);
>> -	pmu->pmc_exists64 = (BIT_ULL(nr_gp_counters) - 1) &
>> -			    (BIT_ULL(kvm_pmu_cap.num_counters_gp) - 1);
>> +	nr_gp_counters = min_t(int, eax.split.num_counters,
>> +			       find_first_zero_bit(kvm_pmu_cap.cntr_mask,
>> +						   KVM_MAX_NR_GP_COUNTERS));
>> +	pmu->pmc_exists64 = BIT_ULL(nr_gp_counters) - 1;
>>  
>>  	entry = kvm_find_cpuid_entry_index(vcpu, 7, 0);