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Re: Commit "x86/kvm: Move context tracking where it belongs" broke guest

From: Wanpeng Li
Subject: Re: Commit "x86/kvm: Move context tracking where it belongs" broke guest time accounting
Date: Wed, 7 Apr 2021 19:01:42 +0800

On Wed, 7 Apr 2021 at 18:55, Michael Tokarev <mjt@tls.msk.ru> wrote:
> Hi!
> It looks like this commit:
> commit 87fa7f3e98a1310ef1ac1900e7ee7f9610a038bc
> Author: Thomas Gleixner <tglx@linutronix.de>
> Date:   Wed Jul 8 21:51:54 2020 +0200
>      x86/kvm: Move context tracking where it belongs
>      Context tracking for KVM happens way too early in the vcpu_run()
>      code. Anything after guest_enter_irqoff() and before guest_exit_irqoff()
>      cannot use RCU and should also be not instrumented.
>      The current way of doing this covers way too much code. Move it closer to
>      the actual vmenter/exit code.
> broke kvm guest cpu time accounting - after this commit, when running
> qemu-system-x86_64 -enable-kvm, the guest time (in /proc/stat and
> elsewhere) is always 0.
> I dunno why it happened, but it happened, and all kernels after 5.9
> are affected by this.
> This commit is found in a (painful) git bisect between kernel 5.8 and 5.10.

Thanks for the report. I have a patch here which can fix it.


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