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From: Michael Tokarev <mjt@tls.msk.ru>
To: kvm@vger.kernel.org, qemu-devel@nongnu.org,
	Thomas Gleixner <tglx@linutronix.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Subject: Commit "x86/kvm: Move context tracking where it belongs" broke guest time accounting
Date: Tue, 6 Apr 2021 23:17:41 +0300	[thread overview]
Message-ID: <36088364-0b3d-d492-0aa4-59ea8f1d1632@msgid.tls.msk.ru> (raw)

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,

/mjt


             reply	other threads:[~2021-04-06 20:24 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-06 20:17 Michael Tokarev [this message]
2021-04-06 21:47 ` Commit "x86/kvm: Move context tracking where it belongs" broke guest time accounting Sean Christopherson
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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