Re: [PATCH v2] KVM: x86/pmu: Don't retry a counter whose config was rejected

[Sandipan Das]

On 20-09-2026 21:24, Luka Absandze wrote:
> kvm_pmu_handle_event() re-arms the reprogram bit for every failed
> reprogram, on the assumption that the failure is transient and a later
> refresh will succeed.  That is true for contention, e.g. the -EBUSY from
> x86_reserve_hardware(), but not for a configuration the host PMU driver
> rejects outright.  A rejected config can never succeed on retry, so the
> counter is reprogrammed on every PMU refresh for as long as the guest
> leaves it enabled, and every attempt fails the same way.
> 
> Re-arm only for the errnos that indicate a transient condition, i.e.
> -EBUSY and -ENOMEM.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/20260916123315.89042-1-absandze@xxxxxxxxx/
> Signed-off-by: Luka Absandze <absandze@xxxxxxxxx>
> ---
> v2:
>  - Use an allow-list of transient errnos (-EBUSY, -ENOMEM) rather than
>    excluding -EINVAL, and hoist 'r' out of the loop.  Diff taken from
>    Sean's suggestion verbatim. [Sean]
>  - Reword the comment to say "fails on a transient condition". [Sean]
>  - v1: https://lore.kernel.org/all/20260918160006.61816-1-absandze@xxxxxxxxx/
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index a7d60c8785cd..03a470c49a74 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -662,7 +662,7 @@ void kvm_pmu_handle_event(struct kvm_vcpu *vcpu)
>  	DECLARE_BITMAP(bitmap, X86_PMC_IDX_MAX);
>  	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>  	struct kvm_pmc *pmc;
> -	int bit;
> +	int bit, r;
>  
>  	bitmap_copy(bitmap, pmu->reprogram_pmi, X86_PMC_IDX_MAX);
>  
> @@ -676,12 +676,14 @@ void kvm_pmu_handle_event(struct kvm_vcpu *vcpu)
>  
>  	kvm_for_each_pmc(pmu, pmc, bit, bitmap) {
>  		/*
> -		 * If reprogramming fails, e.g. due to contention, re-set the
> -		 * reprogram bit, i.e. opportunistically try again on the next
> -		 * PMU refresh.  Don't make a new request as doing so can stall
> -		 * the guest if reprogramming repeatedly fails.
> +		 * If reprogramming fails on a transient condition, e.g. due to
> +		 * contention, re-set the reprogram bit, i.e. opportunistically
> +		 * try again on the next PMU refresh.  Don't make a new request
> +		 * as doing so can stall the guest if reprogramming repeatedly
> +		 * fails.
>  		 */
> -		if (reprogram_counter(pmc))
> +		r = reprogram_counter(pmc);
> +		if (r == -EBUSY || r == -ENOMEM)
>  			set_bit(pmc->idx, pmu->reprogram_pmi);
>  	}
>  

Reviewed-by: Sandipan Das <sandipan.das@xxxxxxx>