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

From: Michael Tokarev
Subject: Commit "x86/kvm: Move context tracking where it belongs" broke guest time accounting
Date: Tue, 6 Apr 2021 23:17:41 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0


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.



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