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Date: Wed, 06 Jul 2011 18:04:06 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: [Qemu-devel] migration: new sections and backward compatibility.
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To: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, jes sorensen <jes.sorensen@redhat.com>, Markus Armbruster <armbru@redhat.com>, Juan Quintela <quintela@redhat.com>

   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.

More cases are lurking.  AHCI doesn't support migration today but 
probably will some day.  Markus mentioned that scsi-disk will face that 
issue too.  And that list probably isn't complete.

Subsections don't help here as there is no toplevel section in the first 
place.

Ideas anyone?  Maybe allow test functions like we have for subsections 
for toplevel sections too, so we have a way to skip the section 
altogether on savevm?

We probably also want a way to fail the migration in case the target 
machine doesn't support migration for $device, especially for $device == 
ahci to avoid data loss.  For the usb-tablet it isn't that problematic, 
in the best case the guest just resets the device and goes on, in the 
worst case the mouse is dead.

Comments?

cheers,
   Gerd