On Wed, Jun 17, 2015 at 06:34:55PM +0200, Juan Quintela wrote: > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > On Wed, Jun 17, 2015 at 02:20:42PM +0200, Paolo Bonzini wrote: > >> > >> > >> On 17/06/2015 14:07, Dr. David Alan Gilbert wrote: > >> > Of course for us on RHEL our minor releases don't correspond to > >> > QEMU minor releases, so we already support migrating from our > >> > downstream 7.1 (QEMU 2.1) derivative to our 7.0 (1.5.3) version. > >> > And the reason for this patch series is to support something >2.2 > >> > migrating back to that 2.1 (or maybe even to that 1.5.3). > >> > > >> > I don't believe we're alone in wanting to be able to do that type > >> > of thing; > >> > >> Others may prefer to have migration only work when it is absolutely sure > >> that it works. It is much easier to add hacks on top of what upstream > >> QEMU does (e.g. using the static checker), than to remove the hacks. > >> > >> If we really didn't care about others' support for bidirectional > >> migration, we would have kept the static checker internal to Red Hat. > >> Or we wouldn't have bothered to refine the .needed functions, and so on. > >> > >> Paolo > > > > What we need to decide is how major is the breakage. > > If it's minor - like some lost characters - then it's not > > worth breaking migration for most users. > > And I think this should be a property so people can > > force strict mode if they really want to. > > > > If it's a major breakage, it's harder to decide: > > some people might be able to retry migration later. > > Maybe a flag to enable this mode would make sense? > > Also, maybe it would be better to fail migration on source > > rather than send something destination can't handle? > > Source don't know if destination understand it or not. That is the > whole point of being optional. Source sends it if it is needed. > Destination can handle it (or not). > > there are (at least) two qemu pc-2.2: > qemu-2.2 -M pc-2.2 > qemu-2.3 -M pc-2.2 > > Same machine type. Second is able to receive it. First one is not. > Source don't know what is on the other side. If user is going to put a: > > --dont_send_serial_because_I_don't_care > > Then it can as well just disable the serial device and live with it. > > Later, Juan.
Just losing worst-case a couple of characters is not the same as losing serial functionality. We could have a flag to tell us what's on the other side, but that would need even more testing. So let's keep it simple. > > > > > But let's see what the symptoms are before we argue > > about this option. > > > >> > so you can either worry about not burdening upstream > >> > with compatibility patches like this, or think it's not fair > >> > to leave them out if others upstream might want them. How many > >> > others? Well I'd say it's got to be more than some of the other > >> > obscure features in QEMU!