Re: [kvm-unit-tests PATCH v2 2/3] x86: Skip running test when pmu is disabled

[Sean Christopherson <seanjc@google.com>]

On Wed, Jun 15, 2022, Yang Weijiang wrote:
> Read MSR_IA32_PERF_CAPABILITIES triggers #GP when pmu is disabled
> by enable_pmu=0 in KVM. Let's check whether pmu is available before
> issue msr reading to avoid the #GP. Also check PDCM bit before read
> the MSR.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> ---
>  x86/pmu_lbr.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/x86/pmu_lbr.c b/x86/pmu_lbr.c
> index 688634d..62614a0 100644
> --- a/x86/pmu_lbr.c
> +++ b/x86/pmu_lbr.c
> @@ -5,6 +5,7 @@
>  #define N 1000000
>  #define MAX_NUM_LBR_ENTRY	  32
>  #define DEBUGCTLMSR_LBR	  (1UL <<  0)
> +#define PDCM_ENABLED	  (1UL << 15)
>  #define PMU_CAP_LBR_FMT	  0x3f
>  
>  #define MSR_LBR_NHM_FROM	0x00000680
> @@ -74,13 +75,22 @@ int main(int ac, char **av)
>  		return 0;
>  	}
>  
> -	perf_cap = rdmsr(MSR_IA32_PERF_CAPABILITIES);
>  	eax.full = id.a;
>  
>  	if (!eax.split.version_id) {
>  		printf("No pmu is detected!\n");
>  		return report_summary();
>  	}
> +
> +	id = cpuid(1);
> +
> +	if (!(id.c & PDCM_ENABLED)) {

Don't open code cpuid(), add and use X86_FEATURE_PDCM:

  #define	X86_FEATURE_PDCM		(CPUID(0x1, 0, ECX, 15))

	if (!this_cpu_has(X86_FEATURE_PDCM))
		...



> +		printf("No PDCM is detected!\n");

If your going to bother printing a message, please make it useful.  Every time I
read PMU code I have to reread the kernel's cpufeatures.h to remember what PDCM
stands for.

		printf("Perf/Debug Capabilities MSR isn't supported\n");

> +		return report_summary();
> +	}
> +
> +	perf_cap = rdmsr(MSR_IA32_PERF_CAPABILITIES);
> +
>  	if (!(perf_cap & PMU_CAP_LBR_FMT)) {
>  		printf("No LBR is detected!\n");

Similar complaint,

		printf("Architectural LBRs are not supported.\n");

>  		return report_summary();
> -- 
> 2.31.1
>