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

[Jim Mattson]

On Tue, Mar 29, 2022 at 6:46 AM Like Xu <like.xu.linux@xxxxxxxxx> wrote:
>
> From: Like Xu <likexu@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx>
> Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>