linux-kernel - Re: [PATCH] perf/x86/kvm: Fix Broadwell Xeon stepping in isolation

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Date:   Wed, 21 Apr 2021 19:05:12 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-kernel@...r.kernel.org, kvm-list@...r.kernel.org,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH] perf/x86/kvm: Fix Broadwell Xeon stepping in
 isolation_ucodes[]

On Wed, Apr 21, 2021 at 05:18:34PM -0700, Jim Mattson wrote:
> The only stepping of Broadwell Xeon parts is stepping 1. Fix the
> relevant isolation_ucodes[] entry, which previously enumerated
> stepping 2.
> 
> Although the original commit was characterized as an optimization, it
> is also a workaround for a correctness issue.
> 
> If a PMI arrives between kvm's call to perf_guest_get_msrs() and the
> subsequent VM-entry, a stale value for the IA32_PEBS_ENABLE MSR may be
> restored at the next VM-exit. This is because, unbeknownst to kvm, PMI
> throttling may clear bits in the IA32_PEBS_ENABLE MSR. CPUs with "PEBS
> isolation" don't suffer from this issue, because perf_guest_get_msrs()
> doesn't report the IA32_PEBS_ENABLE value.

Acked-by: Andi Kleen <ak@...ux.intel.com>

-Andi