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

[Michael S. Tsirkin]

On Mon, Jul 10, 2017 at 10:59:43AM -0300, Eduardo Habkost wrote:
> On Mon, Jul 10, 2017 at 10:42:26AM +0300, Marcel Apfelbaum wrote:
> > On 07/07/2017 21:03, Eduardo Habkost wrote:
> > > 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:
> [...]
> > > > > > 
> > > > > > 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".
> > 
> > We don't really need a complicated rule set, we would just recommend q35
> > by default. Libvirt will try to create the default machine and if fails
> > for some reason (what would it be?) it can switch to PC.
> > 
> > The advanced logic would be "old systems should use PC", where old
> > means Windows XP and before and so on. But this logic should appear
> > in management layers above.
> 
> In this case, is there any difference between "changing the
> default to q35" and "recommending q35", for libvirt users?
> 
> -- 
> Eduardo

No but libvirt users do not manage e.g. pci slots manually.
They do not use the -cdrom flag.
Etc.
So changing the default is unlikely to break things for them.

People not using libvirt use some or all of this
and other such functionality,
and changing the default might break things for them.

-- 
MST