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From: Sean Christopherson <seanjc@google.com>
To: Michael Tokarev <mjt@tls.msk.ru>
Cc: kvm@vger.kernel.org, qemu-devel@nongnu.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: Commit "x86/kvm: Move context tracking where it belongs" broke guest time accounting
Date: Tue, 6 Apr 2021 21:47:40 +0000	[thread overview]
Message-ID: <YGzW/Pa/p7svg5Rr@google.com> (raw)
In-Reply-To: <36088364-0b3d-d492-0aa4-59ea8f1d1632@msgid.tls.msk.ru>

On Tue, Apr 06, 2021, Michael Tokarev 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.

Yes :-(

There's a bugzilla[1] and two proposed fixes[2][3].  I don't particularly like
either of the fixes, but an elegant solution hasn't presented itself.

Thomas/Paolo, can you please weigh in?

[1] https://bugzilla.kernel.org/show_bug.cgi?id=209831
[2] https://lkml.kernel.org/r/1617011036-11734-1-git-send-email-wanpengli@tencent.com
[3] https://lkml.kernel.org/r/20210206004218.312023-1-seanjc@google.com

  reply	other threads:[~2021-04-06 23:48 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-06 20:17 Michael Tokarev
2021-04-06 21:47 ` Sean Christopherson [this message]
2021-04-08 13:18   ` Thomas Gleixner
2021-04-09  2:14     ` Wanpeng Li
2021-04-09  8:13     ` Wanpeng Li
2021-04-09  9:19       ` Thomas Gleixner
2021-04-07 11:01 ` Wanpeng Li
2021-04-13 10:48 ` Wanpeng Li

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