Re: [PATCH V4 1/2] perf ignore LBR and extra_regs.

[Peter Zijlstra]

On Tue, Jul 08, 2014 at 09:49:40AM -0700, kan.li...@intel.com wrote:
> +     /*
> +      * Access LBR MSR may cause #GP under certain circumstances.
> +      * E.g. KVM doesn't support LBR MSR
> +      * Check all LBT MSR here.
> +      * Disable LBR access if any LBR MSRs can not be accessed.
> +      */
> +     if (x86_pmu.lbr_nr) {
> +             access = check_msr(x86_pmu.lbr_tos);
> +             for (i = 0; i < x86_pmu.lbr_nr; i++) {
> +                     access &= check_msr(x86_pmu.lbr_from + i);
> +                     access &= check_msr(x86_pmu.lbr_to + i);
> +             }
> +             if (!access)
> +                     x86_pmu.lbr_nr = 0;
> +     }

You _could_ write that as:

        if (x86_pmu.lbr_nr) {
                if (!check_msr(x86_pmu.lbr_tos))
                        x86_pmu.lbr_nr = 0;
                for (i = 0; i < x86_pmu.lbr_nr; i++) {
                        if (!(check_msr(x86_pmu.lbr_from + i) &&
                              check_msr(x86_pmu.lbr_to + i)))
                              x86_pmu.lbr_nr = 0;
                }
        }

There is no point in checking any more MSRs after the first fail after
all.

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