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Re: [Xen-devel] [PATCH] Fix xenconsole after aborted save

To: Chris Lalancette <clalance@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Fix xenconsole after aborted save
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 22 Apr 2009 11:22:01 +0100
Cc: Jiri Denemark <jdenemar@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Gerd Hoffmann <kraxel@xxxxxxxxxx>
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On 22/04/2009 10:49, "Chris Lalancette" <clalance@xxxxxxxxxx> wrote:

> Keir Fraser wrote:
>> Another thing to note is I think this problem can only occur if the domU
>> does not support suspend cancellation (advertised as SUSPEND_CANCEL in
>> kernel elf notes -- see xen/xend/XendDomainInfo.py:resumeDomain()). Your
>> kernels should support that feature -- suspend cancellation (a.k.a. Resume)
>> is very likely to be hit-or-miss without it!
> Could you elaborate a bit on this?  I was under the impression that suspend
> cancellation was there mostly for the netaccel bits, but I have to admit I
> didn't look at it very closely.  What scenarios do the suspend cancel bits
> help?

Suspend failures (failure to save or to migrate). Also for live
checkpointing/snapshotting. The feature indicates that the guest is happy
for the suspend hypercall to return indicating 'failure/cancelled' and in
which case it can pretty much resume whatever it was doing without any of
the usual resume logic. The alternative is for the toolstack to make it look
like the domain has been restored/migrated, by resetting PV devices and the
like, and this is not much tested and almost inherently fragile.

The suspend_cancel hook you are thinking of is for any PV devices which *do*
need to know that a suspend was cancelled. Netaccel does for some reason
which I cannot recall.

Anyway, it is pretty easy and pretty important to support SUSPEND_CANCEL!

 -- Keir

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