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

[Anthony Liguori]

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