linux-kernel - Re: [Xen-devel] Re: [PATCH] x86/pvclock-xen: zero last

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Date:	Wed, 27 Oct 2010 11:53:18 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Glauber Costa <glommer@...hat.com>
CC:	"Xen-devel@...ts.xensource.com" <Xen-devel@...ts.xensource.com>,
	kvm-devel <kvm@...r.kernel.org>,
	Eelco Dolstra <e.dolstra@...elft.nl>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Olivier Hanesse <olivier.hanesse@...il.com>,
	Avi Kivity <avi@...hat.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] Re: [PATCH] x86/pvclock-xen: zero last_value on resume

 On 10/26/2010 10:48 AM, Glauber Costa wrote:
> On Tue, 2010-10-26 at 09:59 -0700, Jeremy Fitzhardinge wrote:
>> If the guest domain has been suspend/resumed or migrated, then the
>> system clock backing the pvclock clocksource may revert to a smaller
>> value (ie, can be non-monotonic across the migration/save-restore).
>> Make sure we zero last_value in that case so that the domain
>> continues to see clock updates.
>>
>> [ I don't know if kvm needs an analogous fix or not. ]
> After migration, save/restore, etc, we issue an ioctl where we tell
> the host the last clock value. That (in theory) guarantees monotonicity.
>
> I am not opposed to this patch in any way, however.

Thanks.

HPA, do you want to take this, or shall I send it on?

Thanks,
    J
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