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

From: Sean Christopherson <seanjc@google.com>
To: Wanpeng Li <kernellwp@gmail.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>
Subject: Re: [PATCH v2 0/3] KVM: Properly account for guest CPU time
Date: Thu, 15 Apr 2021 00:49:41 +0000	[thread overview]
Message-ID: <YHeNpUd1ZO1JVaAf@google.com> (raw)
In-Reply-To: <CANRm+CzDW_5SPM0131OvRn3UPBp1nahxCykCP61XWeUpYeHU5Q@mail.gmail.com>

On Wed, Apr 14, 2021, Wanpeng Li wrote:
> 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.

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.