Re: [PATCH v2 05/16] KVM: x86/pmu: Add PMC bitmap accessor helpers

[Chen, Zide]

On 10/28/2026 9:32 AM, Jim Mattson wrote:
> On Thu, Aug 27, 2026 at 3:48 PM Zide Chen <zide.chen@xxxxxxxxx> wrote:
>>
>> pmu->nr_arch_{gp,fixed}_counters is not able to represent that a PMU
>> may include non-contiguous GP or fixed counters.
>>
>> pmu->pmc_exists already holds a bitmap indicating both fixed and
>> general-purpose counters, and loops over valid counters can be done via
>> pmu->pmc_exists alone. Extend it to a union so that the u64 alias is
>> available for convenient mask arithmetic operations.
>>
>> Add the necessary helpers to prepare for bitmap-based PMC counter
>> implementation.
>>
>> No functional change intended.
>>
>> Co-developed-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
>> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
>> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
>> ---
>> v2:
>> - Rename kvm_{gp,fixed}_pmc_supported() to kvm_is_{gp,fixed}_pmc_supported()
>>   and simplify them by removing the temporary bitmap. (Sean)
>> - Split kvm_for_each_set_pmc_idx() to kvm_for_each_gp_counter()
>>   and kvm_for_each_fixed_counter(). (Sean)
>> ---
>>  arch/x86/include/asm/kvm_host.h |  5 +++-
>>  arch/x86/kvm/pmu.h              | 53 +++++++++++++++++++++++++++++----
>>  2 files changed, 51 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 90d80483df7f..49a8a2e9cee0 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -594,7 +594,10 @@ struct kvm_pmu {
>>                 DECLARE_BITMAP(reprogram_pmi, X86_PMC_IDX_MAX);
>>                 atomic64_t __reprogram_pmi;
>>         };
>> -       DECLARE_BITMAP(pmc_exists, X86_PMC_IDX_MAX);
>> +       union {
>> +               DECLARE_BITMAP(pmc_exists, X86_PMC_IDX_MAX);
>> +               u64 pmc_exists64;
>> +       };
> 
> Is it worth adding:
> 
> BUILD_BUG_ON(sizeof(pmu->pmc_exists) != sizeof(pmu->pmc_exists64));

Good idea! I can add this:

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 1dac6ae2cbc2..41eaefeea429 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -1051,6 +1051,8 @@ void kvm_pmu_init(struct kvm_vcpu *vcpu)
 {
        struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);

+       BUILD_BUG_ON(sizeof(pmu->pmc_exists) != sizeof(pmu->pmc_exists64));
+
        memset(pmu, 0, sizeof(*pmu));
        kvm_pmu_call(init)(vcpu);
 }

> 
> Either way,
> 
> Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>