linux-kernel - Re: [PATCH v3 00/17] KVM: x86/pmu: Add support to enable Guest PEBS via DS

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Message-ID: <20210115182700.byczztx3vjhsq3p3@two.firstfloor.org>
Date:   Fri, 15 Jan 2021 10:27:00 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Xu, Like" <like.xu@...el.com>, Andi Kleen <andi@...stfloor.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>, eranian@...gle.com,
        kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, wei.w.wang@...el.com,
        luwei.kang@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 00/17] KVM: x86/pmu: Add support to enable Guest PEBS
 via DS

> I'm asking about ucode/hardare.  Is the "guest pebs buffer write -> PEBS PMI"
> guaranteed to be atomic?

Of course not.
> 
> In practice, under what scenarios will guest counters get cross-mapped?  And,
> how does this support affect guest accuracy?  I.e. how bad do things get for the
> guest if we simply disable guest counters if they can't have a 1:1 association
> with their physical counter?

This would completely break perfmon for the guest, likely with no way to
recover.

-Andi