From: Jim Mattson <jmattson@google.com>
To: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: like.xu.linux@gmail.com, eranian@google.com,
santosh.shukla@amd.com, pbonzini@redhat.com, seanjc@google.com,
wanpengli@tencent.com, vkuznets@redhat.com, joro@8bytes.org,
peterz@infradead.org, mingo@redhat.com,
alexander.shishkin@linux.intel.com, tglx@linutronix.de,
bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com,
kvm@vger.kernel.org, x86@kernel.org,
linux-perf-users@vger.kernel.org, ananth.narayan@amd.com,
kim.phillips@amd.com
Subject: Re: [PATCH v2] perf/amd: Implement erratum #1292 workaround for F19h M00-0Fh
Date: Wed, 2 Feb 2022 20:09:05 -0800 [thread overview]
Message-ID: <CALMp9eQHfAgcW-J1YY=01ki4m_YVBBEz6D1T662p2BUp05ZcPQ@mail.gmail.com> (raw)
In-Reply-To: <20220202105158.7072-1-ravi.bangoria@amd.com>
On Wed, Feb 2, 2022 at 2:52 AM Ravi Bangoria <ravi.bangoria@amd.com> wrote:
>
> Perf counter may overcount for a list of Retire Based Events. Implement
> workaround for Zen3 Family 19 Model 00-0F processors as suggested in
> Revision Guide[1]:
>
> To count the non-FP affected PMC events correctly:
> o Use Core::X86::Msr::PERF_CTL2 to count the events, and
> o Program Core::X86::Msr::PERF_CTL2[43] to 1b, and
> o Program Core::X86::Msr::PERF_CTL2[20] to 0b.
>
> Note that the specified workaround applies only to counting events and
> not to sampling events. Thus sampling event will continue functioning
> as is.
>
> Although the issue exists on all previous Zen revisions, the workaround
> is different and thus not included in this patch.
>
> This patch needs Like's patch[2] to make it work on kvm guest.
IIUC, this patch along with Like's patch actually breaks PMU
virtualization for a kvm guest.
Suppose I have some code which counts event 0xC2 [Retired Branch
Instructions] on PMC0 and event 0xC4 [Retired Taken Branch
Instructions] on PMC1. I then divide PMC1 by PMC0 to see what
percentage of my branch instructions are taken. On hardware that
suffers from erratum 1292, both counters may overcount, but if the
inaccuracy is small, then my final result may still be fairly close to
reality.
With these patches, if I run that same code in a kvm guest, it looks
like one of those events will be counted on PMC2 and the other won't
be counted at all. So, when I calculate the percentage of branch
instructions taken, I either get 0 or infinity.
> [1] https://bugzilla.kernel.org/attachment.cgi?id=298241
> [2] https://lore.kernel.org/lkml/20220117055703.52020-1-likexu@tencent.com
>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
next prev parent reply other threads:[~2022-02-03 4:09 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-17 5:57 [PATCH] KVM: x86/pmu: Clear reserved bit PERF_CTL2[43] for AMD erratum 1292 Like Xu
2022-02-02 4:28 ` [PATCH] perf/amd: Implement errata #1292 workaround for F19h M00-0Fh Ravi Bangoria
2022-02-02 5:27 ` Stephane Eranian
2022-02-02 6:02 ` Ravi Bangoria
2022-02-02 6:16 ` Stephane Eranian
2022-02-02 6:32 ` Ravi Bangoria
2022-02-02 10:51 ` [PATCH v2] perf/amd: Implement erratum " Ravi Bangoria
2022-02-02 14:36 ` Peter Zijlstra
2022-02-02 15:32 ` Ravi Bangoria
2022-02-03 9:58 ` [PATCH v3] " Ravi Bangoria
2022-02-09 12:51 ` Peter Zijlstra
2022-02-10 4:05 ` Ravi Bangoria
2022-02-10 8:46 ` Peter Zijlstra
2022-02-03 4:09 ` Jim Mattson [this message]
2022-02-03 5:18 ` [PATCH v2] " Ravi Bangoria
2022-02-03 17:55 ` Jim Mattson
2022-02-04 9:32 ` Ravi Bangoria
2022-02-04 13:01 ` Jim Mattson
2022-02-09 10:18 ` Like Xu
2022-02-09 21:40 ` Jim Mattson
2022-02-10 4:06 ` Ravi Bangoria
2022-02-10 13:56 ` Like Xu
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