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

[Jim Mattson]

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));

Either way,

Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>