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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Fix xenconsole after aborted save
From: Chris Lalancette <clalance@xxxxxxxxxx>
Date: Wed, 22 Apr 2009 16:24:20 +0200
Cc: Jiri Denemark <jdenemar@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Gerd Hoffmann <kraxel@xxxxxxxxxx>
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Keir Fraser wrote:
> 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!

OK, that makes sense.  Thanks Keir!

Chris Lalancette

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