* Paolo Bonzini (pbonz...@redhat.com) wrote: > > > On 17/06/2015 10:37, Dr. David Alan Gilbert wrote: > >> > On 16/06/2015 20:54, Dr. David Alan Gilbert (git) wrote: > >> > > From: "Dr. David Alan Gilbert" <dgilb...@redhat.com> > >> > > > >> > > Older QEMUs dont understand the new (sub)sections that > >> > > may be generated in the serial device. Limit their generation > >> > > to newer machine types. > >> > > > >> > > Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com> > > > > > > No, please. Upstream QEMU doesn't want to get into judgement about when > > > migration quality might be "good enough" that you can drop subsections. > > > > Other people disagree with that statement. > > Who upstream doesn't want it? > > That's always been the policy as far as I know. Certainly I don't. > When we were working on RHEL7, there were quite a few discussions about > this; I remember Orit Wassermann also was a proponent of the "clean > slate" approach.
> The problem is that if you want bug compatibility, you also want a point > (e.g. a major release) where you can start from a clean slate and drop > all compatibility hacks. Upstream there is no such point. It doesn't necessarily have to be a clean slate; the other way to do it is to have a version cut off, so you don't have any bug-compatibility prior to a particular version, and then roll that cut off point forward, much in the same way we do for glib version dependencies. My flag naming of 'serial_migrate_pre_2_2' at least made it clear where the line was for that feature. > It's already hard enough to ensure compatibility of versioned machine > types, which are static, up to QEMU 0.10 or so; imagine what it would be > like to guarantee the same for migration six or seven years down the > line, considering how extremely data-driven migration is. I agree it's not easy, and indeed this serial fix is just one of a whole bunch of fixes that are needed. But if you never try then you never get any compatibility. Dave > > Paolo -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK