Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Jun 15, 2018 at 10:03:14AM +0100, Daniel P. Berrangé wrote:
> On Thu, Jun 14, 2018 at 11:50:56PM -0300, Eduardo Habkost wrote:
> > On Thu, Jun 14, 2018 at 09:09:48AM +0100, Daniel P. Berrangé wrote:
> > > On Wed, Jun 13, 2018 at 03:05:08PM -0300, Eduardo Habkost wrote:
> > > > Getting back to this discussion:
> > > > 
> > > > On Tue, Jun 05, 2018 at 09:43:00AM +0100, Daniel P. Berrangé wrote:
> > > > > On Tue, Jun 05, 2018 at 09:27:46AM +0200, Gerd Hoffmann wrote:
> > > > > >   Hi,
> > > > > > 
> > > > > > > >   Add to that shortcuts like -cdrom
> > > > > > > > stop working,
> > > > > > > 
> > > > > > > Maybe is fixable.
> > > > > > 
> > > > > > Already fixed for ages.
> > > > > > 
> > > > > > > I see marking Q35 as the default machine a first step.
> > > > > > 
> > > > > > Maybe the better option is to go the arm route:  Just don't define a
> > > > > > default, so users have to specify pc or q35.  That will make them notice
> > > > > > there is a world beside 'pc', and we also avoid breaking things
> > > > > > silently.
> > > > > 
> > > > > If QEMU removes the default, then libvirt will have to hardcode
> > > > > 'pc' as the default to maintain back compatibility, so I don't
> > > > > think that ends up as a net win
> > > > 
> > > > I believe there's consensus that applications blindly relying on
> > > > the default machine-type when creating a domain is a bad idea.
> > > > 
> > > > That said, can we deprecate this feature in libvirt, encourage
> > > > applications to always specify an explicit machine-type, thus
> > > > making it possible to deprecate the i440fx machine-types one day?
> > > 
> > > Well from libvirt's POV this scenario arrives if a mgmt app simply omits
> > > the relevant element/attribute from the XML config. Deprecating something
> > > implies that in future we'd drop support for it, but we're never going
> > > to make this mandatory in libvirt as that would be a regression in
> > > behaviour from libvirt's POV. So I don't think it is something we would
> > > deprecate.
> > 
> > Does libvirt really have an option, here?  I'm sure that sooner
> > or later somebody will distribute QEMU binaries without "pc".
> 
> Sure if someone does that, we'll have no choice, but as long as 'pc' is
> shipped we shouldn't gratuitously break apps by changing the default.

Right.  I just want to make sure "omitting the machine-type may
stop working in the future" is documented somehow.

-- 
Eduardo