linux-kernel - Re: [PATCH v2 0/3] KVM: Properly account for guest CPU time

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Date:   Thu, 15 Apr 2021 09:23:34 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Tokarev <mjt@....msk.ru>
Subject: Re: [PATCH v2 0/3] KVM: Properly account for guest CPU time

On Thu, 15 Apr 2021 at 08:49, Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Apr 14, 2021, Wanpeng Li wrote:
> > On Wed, 14 Apr 2021 at 01:25, Sean Christopherson <seanjc@...gle.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.
>
> I fiddled with this a bit today, I think I have something workable that will be
> a relatively clean and short backport.  With luck, I'll get it posted tomorrow.

I think we should improve my posted version instead of posting a lot
of alternative versions to save everybody's time.

    Wanpeng