Re: [PATCH v10 11/29] KVM: x86/pmu: Explicitly check for RDPMC of unsupported Intel PMC types

[Mi, Dapeng]

On 1/10/2024 7:02 AM, Sean Christopherson wrote:
> Explicitly check for attempts to read unsupported PMC types instead of
> letting the bounds check fail.  Functionally, letting the check fail is
> ok, but it's unnecessarily subtle and does a poor job of documenting the
> architectural behavior that KVM is emulating.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/vmx/pmu_intel.c | 21 +++++++++++++++------
>   1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index c37dd3aa056b..b41bdb0a0995 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -26,6 +26,7 @@
>    * further confuse things, non-architectural PMUs use bit 31 as a flag for
>    * "fast" reads, whereas the "type" is an explicit value.
>    */
> +#define INTEL_RDPMC_GP		0
>   #define INTEL_RDPMC_FIXED	INTEL_PMC_FIXED_RDPMC_BASE
>   
>   #define INTEL_RDPMC_TYPE_MASK	GENMASK(31, 16)
> @@ -89,21 +90,29 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu,
>   		return NULL;
>   
>   	/*
> -	 * Fixed PMCs are supported on all architectural PMUs.  Note, KVM only
> -	 * emulates fixed PMCs for PMU v2+, but the flag itself is still valid,
> -	 * i.e. let RDPMC fail due to accessing a non-existent counter.
> +	 * General Purpose (GP) PMCs are supported on all PMUs, and fixed PMCs
> +	 * are supported on all architectural PMUs, i.e. on all virtual PMUs
> +	 * supported by KVM.  Note, KVM only emulates fixed PMCs for PMU v2+,
> +	 * but the type itself is still valid, i.e. let RDPMC fail due to
> +	 * accessing a non-existent counter.  Reject attempts to read all other
> +	 * types, which are unknown/unsupported.
>   	 */
> -	idx &= ~INTEL_RDPMC_FIXED;
> -	if (type == INTEL_RDPMC_FIXED) {
> +	switch (type) {
> +	case INTEL_RDPMC_FIXED:
>   		counters = pmu->fixed_counters;
>   		num_counters = pmu->nr_arch_fixed_counters;
>   		bitmask = pmu->counter_bitmask[KVM_PMC_FIXED];
> -	} else {
> +		break;
> +	case INTEL_RDPMC_GP:
>   		counters = pmu->gp_counters;
>   		num_counters = pmu->nr_arch_gp_counters;
>   		bitmask = pmu->counter_bitmask[KVM_PMC_GP];
> +		break;
> +	default:
> +		return NULL;
>   	}
>   
> +	idx &= INTEL_RDPMC_INDEX_MASK;
>   	if (idx >= num_counters)
>   		return NULL;
>   
Reviewed-by: Dapeng Mi  <dapeng1.mi@xxxxxxxxxxxxxxx>