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Re: [Xen-devel] [PATCH 0/4] HVM Virtual S3

To: "Yu, Ke" <ke.yu@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 0/4] HVM Virtual S3
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Thu, 17 May 2007 08:41:11 +0100
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Thread-topic: [Xen-devel] [PATCH 0/4] HVM Virtual S3
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On 17/5/07 03:32, "Yu, Ke" <ke.yu@xxxxxxxxx> wrote:

> My concern here is that: save/restore is a heavy operation just like S4
> (hibernate), while the purpose of S3 is quick suspend and quick resume
> comapred to S4. if we implement S3 like save/restore, I don't see the value
> here, because HVM save/resotre or HVM S4 is just enough. How do you think?

I do not think that pure virtual S3 by itself makes sense. Unless the whole
machine is going into S3, what really is the difference between a HVM guest
in S3 and an HVM guest that simply is idle and so has all its VCPUs HLTed
99.9% of the time? Both are tying up memory resource, neither is burning
significant CPU resource or I/O resource.

Virtual S3 *does* make sense with HVM save/restore because it makes even an
HVM guest without PV drivers aware of the save/restore event so it can do
things like resync its clock. That is why we are interested in virtual S3:
only as an adjunct to save/restore.

Another application would be as part of host S3. Given that those patches
currently save/restore all domains (which actually I think is stupid, but
that's for a separate email) you should automatically improve host S3 by
integrating with save/restore. However, again, the benefit is not the
effects of the state the guest finds itself in --- since S3 is not
enormously meaningful in a virtualised environment --- but because of the
side effects we enjoy when the guest awakens and resyncs its world.

 -- Keir

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