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Date:   Thu, 14 Jan 2021 11:10:59 -0800
From:   Sean Christopherson <seanjc@google.com>
To:     Like Xu <like.xu@linux.intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        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>,
        Andi Kleen <andi@firstfloor.org>,
        Kan Liang <kan.liang@linux.intel.com>, 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
Message-ID: <YACXQwBPI8OFV1T+@google.com>
References: <20210104131542.495413-1-like.xu@linux.intel.com>
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On Mon, Jan 04, 2021, Like Xu wrote:
> 2) Slow path (part 3, patch 0012-0017)
> 
> This is when the host assigned physical PMC has a different index
> from the virtual PMC (e.g. using physical PMC1 to emulate virtual PMC0)
> In this case, KVM needs to rewrite the PEBS records to change the
> applicable counter indexes to the virtual PMC indexes, which would
> otherwise contain the physical counter index written by PEBS facility,
> and switch the counter reset values to the offset corresponding to
> the physical counter indexes in the DS data structure. 
> 
> Large PEBS needs to be disabled by KVM rewriting the
> pebs_interrupt_threshold filed in DS to only one record in
> the slow path.  This is because a guest may implicitly drain PEBS buffer,
> e.g., context switch. KVM doesn't get a chance to update the PEBS buffer.

Are the PEBS record write, PEBS index update, and subsequent PMI atomic with
respect to instruction execution?  If not, doesn't this approach still leave a
window where the guest could see the wrong counter?

The virtualization hole is also visible if the guest is reading the PEBS records
from a different vCPU, though I assume no sane kernel does that?

> The physical PMC index will confuse the guest. The difficulty comes
> when multiple events get rescheduled inside the guest. Hence disabling
> large PEBS in this case might be an easy and safe way to keep it corrects
> as an initial step here.