[Qemu-devel] new->old version migration

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

New thread stated intentionally, the original patch is Message-ID:
<349e93a4cfc6e1effc1b681cae53f805fdb9624e.1296713825.git.amit.shah@redhat.com>

On Thu, Feb 03, 2011 at 11:47:08AM +0530, Amit Shah wrote:
> Add a compat property for older machine types.  When this is used (via
> -M pc-0.13, for example), the new flow control mechanisms will not be
> used.  This is done to keep migration from a machine started with older
> type on a pc-0.14+ qemu to an older machine working.
> 
> The property is named 'flow_control' and defaults to on.
> 
> Reported-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Amit Shah <amit.shah@redhat.com>

So, I think there are two things that need to be agreed on:

- Can we commit to support migration from new qemu version to an old one?
  We haven't in the past but downstreams do want this,
  so it makes sense to have the infrastructure upstream.

- The infrastructure/command line option for such support.
  We have the -M flags to describe the machine that
  we are running, but that abstracts away guest-visible machine,
  which the migration format is not.
  Also, same qemu could migrate to any older version.
  So I think we would have to add a flag (call it -V for now)
  to savevm/migrate commands to specify the format to be used.
  Naturally some machines would be incompatible with
  specific -V values, that's nothing new.

Pls comment.

-- 
MST