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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Wed, 14 Apr 2021 17:36:14 +0800
Message-ID: <CANRm+CzDW_5SPM0131OvRn3UPBp1nahxCykCP61XWeUpYeHU5Q@mail.gmail.com>
Subject: Re: [PATCH v2 0/3] KVM: Properly account for guest CPU time
To:     Sean Christopherson <seanjc@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Michael Tokarev <mjt@tls.msk.ru>
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On Wed, 14 Apr 2021 at 01:25, Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Apr 13, 2021, Wanpeng Li wrote:
> > The bugzilla https://bugzilla.kernel.org/show_bug.cgi?id=209831
> > reported that the guest time remains 0 when running a while true
> > loop in the guest.
> >
> > The commit 87fa7f3e98a131 ("x86/kvm: Move context tracking where it
> > belongs") moves guest_exit_irqoff() close to vmexit breaks the
> > tick-based time accouting when the ticks that happen after IRQs are
> > disabled are incorrectly accounted to the host/system time. This is
> > because we exit the guest state too early.
> >
> > This patchset splits both context tracking logic and the time accounting
> > logic from guest_enter/exit_irqoff(), keep context tracking around the
> > actual vmentry/exit code, have the virt time specific helpers which
> > can be placed at the proper spots in kvm. In addition, it will not
> > break the world outside of x86.
>
> IMO, this is going in the wrong direction.  Rather than separate context tracking,
> vtime accounting, and KVM logic, this further intertwines the three.  E.g. the
> context tracking code has even more vtime accounting NATIVE vs. GEN vs. TICK
> logic baked into it.
>
> Rather than smush everything into context_tracking.h, I think we can cleanly
> split the context tracking and vtime accounting code into separate pieces, which
> will in turn allow moving the wrapping logic to linux/kvm_host.h.  Once that is
> done, splitting the context tracking and time accounting logic for KVM x86
> becomes a KVM detail as opposed to requiring dedicated logic in the context
> tracking code.
>
> I have untested code that compiles on x86, I'll send an RFC shortly.

We need an easy to backport fix and then we might have some further
cleanups on top.

    Wanpeng