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From: Thomas Gleixner <tglx@linutronix.de>
To: Sean Christopherson <seanjc@google.com>,
	Michael Tokarev <mjt@tls.msk.ru>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	qemu-devel@nongnu.org, kvm@vger.kernel.org
Subject: Re: Commit "x86/kvm: Move context tracking where it belongs" broke guest time accounting
Date: Thu, 08 Apr 2021 15:18:23 +0200	[thread overview]
Message-ID: <874kgg29uo.ffs@nanos.tec.linutronix.de> (raw)
In-Reply-To: <YGzW/Pa/p7svg5Rr@google.com>

On Tue, Apr 06 2021 at 21:47, Sean Christopherson wrote:
> On Tue, Apr 06, 2021, Michael Tokarev wrote:
>> 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

All of the solutions I looked at so far are ugly as hell. The problem is
that the accounting is plumbed into the context tracking and moving
context tracking around to a different place is just wrong.

I think the right solution is to seperate the time accounting logic out
from guest_enter/exit_irqoff() and have virt time specific helpers which
can be placed at the proper spots in kvm.

Thanks,

        tglx


  reply	other threads:[~2021-04-08 13:19 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
2021-04-08 13:18   ` Thomas Gleixner [this message]
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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