Re: [kvm-unit-tests PATCH v3 08/13] x86/pmu: Add PDCM check before accessing PERF_CAP register

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 19, 2022, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> On virtual platforms without PDCM support (e.g. AMD), #GP
> failure on MSR_IA32_PERF_CAPABILITIES is completely avoidable.
> 
> Signed-off-by: Like Xu <likexu@tencent.com>
> ---
>  x86/pmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/x86/pmu.c b/x86/pmu.c
> index 4eb92d8..25fafbe 100644
> --- a/x86/pmu.c
> +++ b/x86/pmu.c
> @@ -669,7 +669,8 @@ int main(int ac, char **av)
>  
>  	check_counters();
>  
> -	if (rdmsr(MSR_IA32_PERF_CAPABILITIES) & PMU_CAP_FW_WRITES) {
> +	if (this_cpu_has(X86_FEATURE_PDCM) &&
> +	    (rdmsr(MSR_IA32_PERF_CAPABILITIES) & PMU_CAP_FW_WRITES)) {


pmu_pebs.c ends up with similar code.  Maybe add a helper?

  static inline u64 this_cpu_perf_capabilities(void)
  {
	if (!this_cpu_has(X86_FEATURE_PDCM))
		return 0;

	rdmsr(MSR_IA32_PERF_CAPABILITIES);
  }

Or even better, if we end up with a lib/pmu and something like "struct pmu_caps",
the RDMSR can be done once in the library's init routine.