On Sun, Apr 08, 2012 at 08:30:47AM -0400, Ayal Baron wrote: > > > ----- Original Message ----- > > N.B. This is a small patch with significant implications. Please > > read > > carefully. > > > > Right now, '-M pc' is the default and, in general, this machine type > > has guest > > visible ABI changes in each version of QEMU. At some point in each > > release, > > we create a '-M pc-X.Y' corresponding to the last release and attempt > > to make > > that look like the previous QEMU version's machine. > > > > We've accumulated a bunch of '-M pc-X.Y's at this point and if we > > move to a > > three month release process, the problem will get much worse. > > I agree that every 3 months seems excessive but moving from this to every 24 > months seems to me to be taking it too far (and would cause too many > problems). > There is a balance to be struck here and I think it should be somewhere > between 6 and 12 months. > Take for example a qemu released 6 month before V2.0 and another one released > 3 months before V2.0, assuming the latter has some changes in it so that > pc-next means something else, in order to be able to migrate a VM between > them I'd need to use -M pc1.0 which would be at least 21 months old and not > be able to use any of the features introduced during this time.
Indeed, this does seem like rather a long gap between versions. I can understand the desire to minimize the number of machine type versions we need to support, but this does seem like going a bit too far in the other direction > I have little doubt this would not hold water downstream where we'd have to > have a compatibility version per minor RHEL release. I presume that regardless of what QEMU does here, RHEL will still end up defining its own machine types & ignoring QEMUs, due to the need to backport features from across releases. Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
