linux-kernel - Re: [PATCH] perf/x86/intel: KVM: Mask PEBS_ENABLE loaded for guest with vCPU's value.

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Message-ID: <aBTe6dpaQs6bmFwh@google.com>
Date: Fri, 2 May 2025 08:04:09 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Seth Forshee <sforshee@...nel.org>
Subject: Re: [PATCH] perf/x86/intel: KVM: Mask PEBS_ENABLE loaded for guest
 with vCPU's value.

On Sun, Apr 27, 2025, Dapeng Mi wrote:
> On 4/26/2025 8:13 AM, Sean Christopherson wrote:
> Currently we have this Sean's fix, only the guest PEBS event bits of
> IA32_PEBS_ENABLE MSR are enabled in non-root mode, suppose we can simply
> change global_ctrl guest value calculation to this.
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 09d2d66c9f21..5bc56bb616ec 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4342,9 +4342,12 @@ static struct perf_guest_switch_msr
> *intel_guest_get_msrs(int *nr, void *data)
>         arr[global_ctrl] = (struct perf_guest_switch_msr){
>                 .msr = MSR_CORE_PERF_GLOBAL_CTRL,
>                 .host = intel_ctrl & ~cpuc->intel_ctrl_guest_mask,
> -               .guest = intel_ctrl & ~cpuc->intel_ctrl_host_mask & ~pebs_mask,
> +               .guest = intel_ctrl & ~cpuc->intel_ctrl_host_mask,
>         };

Hmm, that's not as clear cut.  PEBS needs to be disabled because leaving it enabled
will crash the guest.  For the counter itself, unless leaving it enabled breaks
perf and/or degrades the sampling, I don't think there's an obvious right/wrong
approach.

E.g. if the host wants to profile host and guest, then keeping the count running
while the guest is active might be a good thing.  It's still far, far from
perfect, as a counter that overflows when the guest is active won't generate a
PEBS record, but without digging further, it's not obvious that even that flaw
is overall worse than always disabling the counter.