Re: [PATCH V2 1/4] KVM: x86/pmu: Do not map fixed counters >= 3 to generic perf events

[Mi, Dapeng]

On 4/24/2026 1:46 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.
>
> Also remove __always_inline as BUILD_BUG_ON() is no longer needed.
>
> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> ---
> V2:
> - Replace 3 in "if (index < 3)" with ARRAY_SIZE(fixed_pmc_perf_ids).
> ---
>  arch/x86/kvm/vmx/pmu_intel.c | 26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 27eb76e6b6a0..05a59f4acfdd 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -454,28 +454,30 @@ 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,
>  		[1] = PERF_COUNT_HW_CPU_CYCLES,
>  		[2] = PERF_COUNT_HW_REF_CPU_CYCLES,
>  	};
> -	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);
> +	u64 eventsel = 0;
>  
>  	/*
> -	 * Yell if perf reports support for a fixed counter but perf doesn't
> -	 * have a known encoding for the associated general purpose event.
> +	 * Fixed counters 3 and above don't have corresponding generic hardware
> +	 * perf event, and KVM does not intend to emulate them on non-mediated
> +	 * vPMU.

The above comments are ambiguous. Fixed counter 3 and above indeed have
dedicated perf event, but they are not supported perf_hw_id{} yet. So KVM
has no way to get their specific event code now.

Thanks.


>  	 */
> -	eventsel = perf_get_hw_event_config(fixed_pmc_perf_ids[index]);
> -	WARN_ON_ONCE(!eventsel && index < kvm_pmu_cap.num_counters_fixed);
> +	if (index < ARRAY_SIZE(fixed_pmc_perf_ids)) {
> +		/*
> +		 * Yell if perf reports support for a fixed counter but perf
> +		 * doesn't have a known encoding for the associated general
> +		 * purpose event.
> +		 */
> +		eventsel = perf_get_hw_event_config(fixed_pmc_perf_ids[index]);
> +		WARN_ON_ONCE(!eventsel && index < kvm_pmu_cap.num_counters_fixed);
> +	}
>  	return eventsel;
>  }
>