Re: [PATCH v2 12/16] KVM: x86/pmu: Populate CPUID.0AH:ECX fixed-counter bitmap

[Jim Mattson]

On Thu, Aug 27, 2026 at 3:48 PM Zide Chen <zide.chen@xxxxxxxxx> 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>
> ---
>  arch/x86/kvm/cpuid.c         | 5 ++++-
>  arch/x86/kvm/vmx/pmu_intel.c | 3 +++
>  2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 106e719e7aee..63201d4ea4a1 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1547,7 +1547,10 @@ 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;

Is the '&' necessary?