Re: [Qemu-devel] [PATCH] Migration compatibility for serial

[Paolo Bonzini <pbonzini@redhat.com>]

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" <dgilbert@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 <dgilbert@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'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.

Paolo