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

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Message-ID: <6f805099c5751a3092ee5f198fffb83673ba91ee.camel@infradead.org>
Date: Tue, 20 Aug 2024 11:22:31 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: 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>,
 Steven Rostedt <rostedt@...dmis.org>, 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 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.

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