Re: [PATCH v4 09/16] KVM: x86/pmu: Add PEBS_DATA_CFG MSR emulation to support adaptive PEBS

[Peter Zijlstra]

On Mon, Mar 29, 2021 at 01:41:30PM +0800, Like Xu wrote:
> @@ -3863,6 +3864,12 @@ static struct perf_guest_switch_msr 
> *intel_guest_get_msrs(int *nr, void *data)
>               arr[2].host = (unsigned long)ds;
>               arr[2].guest = pmu->ds_area;

                *nr = 3;
>  
> +             if (baseline) {
> +                     arr[3].msr = MSR_PEBS_DATA_CFG;
> +                     arr[3].host = cpuc->pebs_data_cfg;
> +                     arr[3].guest = pmu->pebs_data_cfg;

                        *nr = 4;
> +             }
> +
>               /*
>                * If PMU counter has PEBS enabled it is not enough to
>                * disable counter on a guest entry since PEBS memory
> @@ -3879,9 +3886,11 @@ static struct perf_guest_switch_msr 
> *intel_guest_get_msrs(int *nr, void *data)
>               else {
>                       arr[1].guest = arr[1].host;
>                       arr[2].guest = arr[2].host;
> +                     if (baseline)
> +                             arr[3].guest = arr[3].host;
>               }
>  
> -             *nr = 3;
> +             *nr = baseline ? 4 : 3;

And you don't need yet another branch to determine a value you already
know.

>       }