linux-kernel - Re: [PATCH] time,virt: resync steal time when guest & host lose sync

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Message-ID: <c5405116-91ca-23cc-6855-0b9109f1d55d@redhat.com>
Date:	Fri, 12 Aug 2016 18:33:54 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Rik van Riel <riel@...hat.com>, Wanpeng Li <kernellwp@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] time,virt: resync steal time when guest & host lose sync

On 10/08/2016 18:52, Rik van Riel wrote:
> Paolo, what is your opinion on this issue?
> 
> I can think of all kinds of ways in which guest and host might lose
> sync with steal time, from uninitialized values at boot, to guest
> pause, followed by save to disk, and reload, to live migration, to...

Guest and host _cannot_ lose sync because there is only one copy of the
values.  When the host wants to update the steal time value it just
reads the old value and writes the new value.  There cannot be a guest
pause, save to disk, live migration or whatever between these two steps
(and uninitialized values at boot are not how percpu values work).

Your hypothesis of lost ticks makes the most sense to me, and then
changing the argument to ULONG_MAX is the right thing to do.

Paolo