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Date: Thu, 07 Jul 2011 09:19:06 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] migration: new sections and backward compatibility.
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: jes sorensen <jes.sorensen@redhat.com>, Juan Quintela <quintela@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>

On 07/06/11 19:13, Anthony Liguori wrote:
> On 07/06/2011 11:04 AM, Gerd Hoffmann wrote:
>> Hi folks,
>>
>> We'll need to figure a sane way to handle migration to older versions
>> with new sections, i.e. devices which used to not save state before do
>> now.
>>
>> We already have one case in tree: usb. qemu 0.14 saves state for usb-hid
>> devices and the usb-hub, whereas qemu 0.13 and older don't. You can't
>> migrate a vm with a usb-tablet from 0.14 to 0.13 because of that even if
>> you use -M pc-0.13.
>
> Because if you did migrate, you would actively break the guest during
> migration. So why is this a problem?

Well, in case of usb hid devices breaking the guest isn't that a big 
issue for at least some guests because they manage to reset the device 
and continue nevertheless ...

I think this is a case-by-case thing.  In some cases we want break 
migration because critical state is missing.  In other cases we might 
want allow it nevertheless.

cheers,
   Gerd