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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] migration: new sections and backward compatibility. |
Date: | Thu, 07 Jul 2011 10:36:32 -0500 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110424 Lightning/1.0b2 Thunderbird/3.1.10 |
On 07/07/2011 02:19 AM, Gerd Hoffmann wrote:
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 ...
In a situation like this, I think our responsibility is to let the user know that there could be a problem, and provide the ability to the user to force the migration.
So for instance, you could have a "(qemu) migrate_ignore_section usb" command or something like that.
But we shouldn't enable things that may sometimes work by default. Regards, Anthony Liguori
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
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