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

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

On Mon, Feb 07, 2011 at 01:33:57PM -0600, Anthony Liguori wrote:
> On 02/07/2011 10:07 AM, Michael S. Tsirkin wrote:
> >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.
> 
> Only within a stable release series and only when it's possible
> without sacrificing integrity.  I know some downstreams disagree
> with this but I don't think this is a business we want to get into.
> 
> Regards,
> 
> Anthony Liguori
> >- 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.

OK, assuming we want this, let's talk about implementation.
I think that spreading custom flags all over the code like
this patch does would be pretty bad.

What I'd like to see is a way to
- map stable versions (e.g. machine type if we are going
  to tie to that)  to savevm format using
  some kind of table
- for save callbacks to be able to figure out what
  version to use

-- 
MST