Re: [PATCH] KVM: arm64: Use raw_smp_processor_id() in kvm_pmu_probe_armpmu()

[Sean Christopherson <seanjc@google.com>]

On Tue, Jun 06, 2023, Oliver Upton wrote:
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index 491ca7eb2a4c..560650972478 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -700,7 +700,25 @@ static struct arm_pmu *kvm_pmu_probe_armpmu(void)
>  
>  	mutex_lock(&arm_pmus_lock);
>  
> -	cpu = smp_processor_id();
> +	/*
> +	 * It is safe to use a stale cpu to iterate the list of PMUs so long as
> +	 * the same value is used for the entirety of the loop. Given this, and
> +	 * the fact that no percpu data is used for the lookup there is no need
> +	 * to disable preemption.
> +	 *
> +	 * It is still necessary to get a valid cpu, though, to probe for the
> +	 * default PMU instance as userspace is not required to specify a PMU
> +	 * type. In order to uphold the preexisting behavior KVM selects the
> +	 * PMU instance for the core where the first call to the
> +	 * KVM_ARM_VCPU_PMU_V3_CTRL attribute group occurs. A dependent use case
> +	 * would be a user with disdain of all things big.LITTLE that affines
> +	 * the VMM to a particular cluster of cores.
> +	 *
> +	 * In any case, userspace should just do the sane thing and use the UAPI
> +	 * to select a PMU type directly. But, be wary of the baggage being
> +	 * carried here.
> +	 */

Thanks for humoring me :-)