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Re: [Xen-devel] Re: [PATCH] x86/pvclock-xen: zero last_value on resume

To: Glauber Costa <glommer@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] x86/pvclock-xen: zero last_value on resume
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 27 Oct 2010 11:53:18 -0700
Cc: "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>, kvm-devel <kvm@xxxxxxxxxxxxxxx>, Eelco Dolstra <e.dolstra@xxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Linux Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>, Olivier Hanesse <olivier.hanesse@xxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>
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 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.


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


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