linux-kernel - Re: [PATCH] perf/x86/intel: Save/restore cpuc->active_pebs_data

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Date:   Thu, 18 May 2023 12:31:12 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Like Xu <like.xu.linux@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf/x86/intel: Save/restore cpuc->active_pebs_data_cfg
 when using guest PEBS



On 2023-05-17 9:38 a.m., Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> After commit b752ea0c28e3 ("perf/x86/intel/ds: Flush PEBS DS when changing
> PEBS_DATA_CFG"), the cpuc->pebs_data_cfg may save some bits that are not
> supported by real hardware, such as PEBS_UPDATE_DS_SW. This would cause
> the VMX hardware MSR switching mechanism to save/restore invalid values
> for PEBS_DATA_CFG MSR, thus crashing the host when PEBS is used for guest.

I believe we clear the SW bit when it takes effect.

+	if (cpuc->pebs_data_cfg & PEBS_UPDATE_DS_SW) {
+		cpuc->pebs_data_cfg = pebs_data_cfg;
+		pebs_update_threshold(cpuc);
+	}

I think the SW bit can only be seen in a shot period between add() and
enable(). Is it caused by a VM enter which just happens on the period?

> Fix it by using the active host value from cpuc->active_pebs_data_cfg.

I don't see a problem of using active_pebs_data_cfg, since it reflects
the current MSR setting. Just curious about how it's triggered.

> 
> Cc: Kan Liang <kan.liang@...ux.intel.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---

Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>

Thanks,
Kan

>  arch/x86/events/intel/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 070cc4ef2672..89b9c1cebb61 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4074,7 +4074,7 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
>  	if (x86_pmu.intel_cap.pebs_baseline) {
>  		arr[(*nr)++] = (struct perf_guest_switch_msr){
>  			.msr = MSR_PEBS_DATA_CFG,
> -			.host = cpuc->pebs_data_cfg,
> +			.host = cpuc->active_pebs_data_cfg,
>  			.guest = kvm_pmu->pebs_data_cfg,
>  		};
>  	}