Re: [PATCH v6 1/8] KVM: x86/pmu: Do not map fixed counters >= 3 to generic perf events

[Mi, Dapeng]

LGTM. Thanks.

Reviewed-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>

On 6/30/2026 7:19 AM, Zide Chen wrote:
> Only fixed counters 0..2 have matching generic cross-platform
> hardware perf events (INSTRUCTIONS, CPU_CYCLES, REF_CPU_CYCLES).
> Therefore, perf_get_hw_event_config() is only applicable to these
> counters.
>
> KVM does not intend to emulate fixed counters >= 3 on legacy
> (non-mediated) vPMU, while for mediated vPMU, KVM does not care what
> the fixed counter event mappings are.  Therefore, return 0 for their
> eventsel.
>
> The two BUILD_BUG_ON() checks are no longer needed, so drop them along
> with __always_inline.
>
> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> ---
> v6:
> - Re-arrange the code for early return. Clearer.
> v2:
> - Replace 3 in "if (index < 3)" with ARRAY_SIZE(fixed_pmc_perf_ids).
> ---
>  arch/x86/kvm/vmx/pmu_intel.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index a73a9515d96c..f15af497d27f 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -464,11 +464,8 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   * different perf_event is already utilizing the requested counter, but the end
>   * result is the same (ignoring the fact that using a general purpose counter
>   * will likely exacerbate counter contention).
> - *
> - * Forcibly inlined to allow asserting on @index at build time, and there should
> - * never be more than one user.
>   */
> -static __always_inline u64 intel_get_fixed_pmc_eventsel(unsigned int index)
> +static u64 intel_get_fixed_pmc_eventsel(unsigned int index)
>  {
>  	const enum perf_hw_id fixed_pmc_perf_ids[] = {
>  		[0] = PERF_COUNT_HW_INSTRUCTIONS,
> @@ -477,8 +474,13 @@ static __always_inline u64 intel_get_fixed_pmc_eventsel(unsigned int index)
>  	};
>  	u64 eventsel;
>  
> -	BUILD_BUG_ON(ARRAY_SIZE(fixed_pmc_perf_ids) != KVM_MAX_NR_INTEL_FIXED_COUNTERS);
> -	BUILD_BUG_ON(index >= KVM_MAX_NR_INTEL_FIXED_COUNTERS);
> +	/*
> +	 * Fixed counters 3 and above don't have a corresponding generic
> +	 * hardware perf event, and KVM does not intend to emulate them on
> +	 * non-mediated vPMU.
> +	 */
> +	if (index >= ARRAY_SIZE(fixed_pmc_perf_ids))
> +		return 0;
>  
>  	/*
>  	 * Yell if perf reports support for a fixed counter but perf doesn't