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

[Eduardo Habkost]

On Fri, Jul 07, 2017 at 06:17:57PM +0300, Michael S. Tsirkin wrote:
> On Fri, Jul 07, 2017 at 10:39:49AM -0300, Eduardo Habkost wrote:
> > On Wed, Jul 05, 2017 at 12:32:10PM +0300, Marcel Apfelbaum wrote:
> > > On 05/07/2017 11:14, Thomas Huth wrote:
> > > >   Hi,
> > > > 
> > > 
> > > 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.
> > > > 
> > > 
> > > Agreed
> > > 
> > > > Yes, IMHO at one point in time, we should switch the default machine
> > > > type to q35.
> > > 
> > > +1
> > > 
> > > > 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 ...
> > > 
> > > I am not so sure about that. Developers working on upstream projects
> > > should expect such changes and, for our case,
> > > modifying the command line by adding "-M pc" should not be a big deal.
> > 
> > We could print a warning for 1 or 2 releases when users don't add
> > a explicit "-M pc" or "-M q35" argument to qemu-system-x86_64,
> > but:
> > 
> > > 
> > > The upper layers should manage the defaults by themselves so
> > > are not supposed to be affected.
> > 
> > But they would be.  libvirt uses the default machine-type from
> > QEMU.
> 
> How about extending the command for supported machines with a
> recommended machine type, and teaching libvirt to use that?

I don't think QEMU has enough information to decide if it should
recommend "q35" or "pc".

-- 
Eduardo