Re: [Qemu-devel] change x86 default machine type to Q35?

[Stefan Hajnoczi]

On Wed, Jul 05, 2017 at 12:07:57PM +0100, Daniel P. Berrange wrote:
> On Wed, Jul 05, 2017 at 10:14:23AM +0200, Thomas Huth wrote:
> >  Hi,
> > 
> > On 05.07.2017 08:57, Chao Peng wrote:
> > > 
> > > Q35 has been in QEMU for quite a while. Compared to the current default
> > > i440FX, Q35 is probably not that mature and not widely used, however in
> > > some case, Q35 has advantages, for example, in supporting new features.
> > > For instance, we have some features require PCI-e support which is only
> > > available on Q35 and some others need it for EFI support. It is of
> > > course not necessary to change it as the default but if more and more
> > > features have dependencies on Q35 because of requiring much more modern
> > > features then I think it may be worth to do so. In such case we can have
> > > more people to use it and find problems we may know or not know.
> > 
> > Yes, IMHO at one point in time, we should switch the default machine
> > type to q35. The i440FX is really quite old...
> > 
> > > There are certainly some drawbacks:
> > > -        Compatibility: current code or script may need adjustment
> > 
> > That might be a real concern ... so I think a good point in time to
> > switch to the q35 machine type would be when we switch to the next major
> > version number of QEMU, i.e. when we switch to version 3.0. If the users
> > see a new major version number, they might be more willing to accept
> > such major changes (yeah, I know, we've discussed in the past that
> > version numbers are just numbers ... but still, there is some kind of
> > psychological aspect to this, too, I think)
> 
> Most users aren't even aware of version numbers of QEMU - they'll just
> take whatever their distro has provided run it. The notion that their
> latest distro version happened to pull in a "major" version instead of
> previously pulling in a "minor" version is invisible to everyone,
> except the minority of people who care about the low level details.

When user-visible changes break existing setups it's common for
packagers to ship a new family of packages that includes the major
version number (e.g. apache2, postgresql-9.6).

The last QEMU 2.x version would be considered stable and still available
from package repos.  Only critical bug and security fixes could be
released for another, say, 2 years.

This way users don't have to migrate to QEMU 3.x until they are ready.

Stefan

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