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

[Avi Kivity]

On 07/06/2011 07:04 PM, 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.
>
> 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.

How did AHCI get in without migration?  It's relatively new, is it not?

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