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

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [PATCH] x86/pvclock-xen: zero last_value on resume
From: Glauber Costa <glommer@xxxxxxxxxx>
Date: Tue, 26 Oct 2010 15:48:54 -0200
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 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.

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