linux-kernel - Re: [PATCH v2 12/12] KVM: x86/pmu: Clear reserved bit PERF

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Date:   Fri, 4 Mar 2022 17:46:58 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 12/12] KVM: x86/pmu: Clear reserved bit PERF_CTL2[43]
 for AMD erratum 1292

On 3/3/2022 1:52 am, Jim Mattson wrote:
> On Wed, Mar 2, 2022 at 3:14 AM Like Xu <like.xu.linux@...il.com> wrote:
>>
>> From: Like Xu <likexu@...cent.com>
>>
>> The AMD Family 19h Models 00h-0Fh Processors may experience sampling
>> inaccuracies that cause the following performance counters to overcount
>> retire-based events. To count the non-FP affected PMC events correctly,
>> a patched guest with a target vCPU model would:
>>
>>      - Use Core::X86::Msr::PERF_CTL2 to count the events, and
>>      - Program Core::X86::Msr::PERF_CTL2[43] to 1b, and
>>      - Program Core::X86::Msr::PERF_CTL2[20] to 0b.
>>
>> To support this use of AMD guests, KVM should not reserve bit 43
>> only for counter #2. Treatment of other cases remains unchanged.
>>
>> AMD hardware team clarified that the conditions under which the
>> overcounting can happen, is quite rare. This change may make those
>> PMU driver developers who have read errata #1292 less disappointed.
>>
>> Reported-by: Jim Mattson <jmattson@...gle.com>
>> Signed-off-by: Like Xu <likexu@...cent.com>
> 
> This seems unnecessarily convoluted. As I've said previously, KVM
> should not ever synthesize a #GP for any value written to a
> PerfEvtSeln MSR when emulating an AMD CPU.

IMO, we should "never synthesize a #GP" for all AMD MSRs,
not just for AMD PMU msrs, or keep the status quo.

I agree with you on this AMD #GP transition, but we need at least one
kernel cycle to make a more radical change and we don't know Paolo's
attitude and more, we haven't received a tidal wave of user complaints.