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Date:   Fri, 22 Jan 2021 10:56:12 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Andi Kleen <andi@firstfloor.org>, "Xu, Like" <like.xu@intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, eranian@google.com,
        kvm@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, wei.w.wang@intel.com,
        luwei.kang@intel.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 00/17] KVM: x86/pmu: Add support to enable Guest PEBS
 via DS
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On Fri, Jan 15, 2021 at 10:51:38AM -0800, Sean Christopherson wrote:
> On Fri, Jan 15, 2021, Andi Kleen wrote:
> > > I'm asking about ucode/hardare.  Is the "guest pebs buffer write -> PEBS PMI"
> > > guaranteed to be atomic?
> > 
> > Of course not.
> 
> So there's still a window where the guest could observe the bad counter index,
> correct?

Guest could do a hypercall to fix up the DS area before it tries to read
it I suppose. Or the HV could expose the index mapping and have the
guest fix up it.

Adding a little virt crud on top shouldn't be too hard.