linux-kernel - Re: [RFC PATCH v3 20/21] KVM: x86/xen: Prevent runstate times from becoming negative

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Message-ID: <20240820110814.7d4117fd@gandalf.local.home>
Date: Tue, 20 Aug 2024 11:08:14 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org, Paolo
 Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, Thomas
 Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav
 Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Paul Durrant
 <paul@....org>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli
 <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall
 <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Daniel Bristot de
 Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, jalliste@...zon.co.uk, sveith@...zon.de,
 zide.chen@...el.com, Dongli Zhang <dongli.zhang@...cle.com>, Chenyi Qiang
 <chenyi.qiang@...el.com>
Subject: Re: [RFC PATCH v3 20/21] KVM: x86/xen: Prevent runstate times from
 becoming negative

On Tue, 20 Aug 2024 11:22:31 +0100
David Woodhouse <dwmw2@...radead.org> wrote:

> On Thu, 2024-08-15 at 21:39 -0700, Sean Christopherson wrote:
> > > +       vx->last_steal = run_delay;
> > > +
> > > +       /*
> > > +        * If KVM clock time went backwards, stop updating until it
> > > +        * catches up (or the runstates are reset by userspace).
> > > +        */  
> > 
> > I take it this is a legitimate scenario where userpace sets KVM clock and then
> > the runstates, and KVM needs to lend a hand because userspace can't do those two
> > things atomically?  
> 
> Indeed. Will update the comment to make that more obvious.
> 
> Thanks for the rest of the review on this series. I'll go through in
> detail and update it, hopefully this week.

Hmm, is this related at all to this:

  https://lore.kernel.org/all/20240806111157.1336532-1-suleiman@google.com/

-- Steve