linux-kernel - Re: [PATCH] KVM: x86/pmu: Update AMD PMC smaple period to fix guest NMI-watchdog

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Date:   Fri, 1 Apr 2022 11:26:31 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Eric Hankland <ehankland@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/pmu: Update AMD PMC smaple period to fix guest NMI-watchdog

On Tue, Mar 29, 2022 at 6:46 AM Like Xu <like.xu.linux@...il.com> wrote:
>
> From: Like Xu <likexu@...cent.com>
>
> NMI-watchdog is one of the favorite features of kernel developers,
> but it does not work in AMD guest even with vPMU enabled and worse,
> the system misrepresents this capability via /proc.
>
> This is a PMC emulation error. KVM does not pass the latest valid
> value to perf_event in time when guest NMI-watchdog is running, thus
> the perf_event corresponding to the watchdog counter will enter the
> old state at some point after the first guest NMI injection, forcing
> the hardware register PMC0 to be constantly written to 0x800000000001.
>
> Meanwhile, the running counter should accurately reflect its new value
> based on the latest coordinated pmc->counter (from vPMC's point of view)
> rather than the value written directly by the guest.
>
> Fixes: 168d918f2643 ("KVM: x86: Adjust counter sample period after a wrmsr")
> Reported-by: Dongli Cao <caodongli@...gsoft.com>
> Signed-off-by: Like Xu <likexu@...cent.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>