[Qemu-devel] Re: new->old version migration

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Feb 07, 2011 at 08:23:08PM -0600, Anthony Liguori wrote:
> On 02/07/2011 03:52 PM, Michael S. Tsirkin wrote:
> >How does it? We need to know we are saving in 0.13
> >format and skip the new subsection, otherwise
> >0.13 will see a subsection it does not recognize
> >and exit.
> 
> If you used subsections for flow control, presumably you would only
> send the new savevm data if you had data buffered.
> 
> If you add a qdev property to enable/disable flow control, then if
> it's disabled, you naturally would never send the subsection because
> you'd never buffer data.  So no explicit code is needed to support
> migration.

But the result is we get a new property that we can never remove
as any qdev property is part of interface.

> The difficult case is when you truly need to change the savevm
> version.  I don't think we have a proper fix for this because
> versions are linear so the proposed patch certainly wouldn't be a
> good way to do it.  if flow_control=0 causes savevm 3 to be used
> instead of 4, and then the next_feature=0 causes savevm 4 to be used
> instead of 5, the semantics of flow_control=0,next_feature=1 becomes
> problematic.
> 
> But as long as the feature has isolated state, we can solve the
> problem robustly with subsections.
> 
> Regards,
> 
> Anthony Liguori

I see. I'm unhappy with the facts that
1. if (feature) is spread all over the code instead
   of just in migration
2. it is also obfuscated with if (flow_control)
   instead of plain if (migrate to qemu < 0.14)
   so removing it will be much harder
3. this forces anyone who wants
   a VM compatible with qemu 0.13 to also lose data,
   even if migration to 0.13 is never attempted.

> >We also need API to add subsections without vmstate,
> >because virtio serial wasn't yet converted.
> >