Re: [Qemu-devel] migration: new sections and backward compatibility.

[Gerd Hoffmann]

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