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

[Marcel Apfelbaum]

On 06/05/2018 04:29 PM, Daniel P. Berrangé wrote:
> On Tue, Jun 05, 2018 at 04:20:46PM +0300, Marcel Apfelbaum wrote:
> > On 06/05/2018 11:43 AM, 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.
> > It can work, sure. And we can add user hints: "Use q35 for ...., select pc
> > if..."
> >
> > > 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
> > Can't libvirt preserve 'pc' for existing domains, while defaulting to q35
> > the creation of new domains ? This way it aligns with Gerd's proposal of no
> > default x86 machine.
> Existing domains wasn't the case I was concerned about. Consider you have
> libvirt 4.4.0 intsalled and you deploy a *new* domain from a prebuilt
> disk image "foo".

Using Laszlo idea (I think):
 1) disk image "foo"/no info -> use 'pc' machine.
 2) no prebuild image (clean install?) ->  use q35 and add this info to 
xml
In the meantime find a way to embed the supported machine type(s) in the 
image.
(I think the above idea was already discussed, not sure what was the 
conclusion )


Thanks,
Marcel

>   Now update to a libvirt or QEMU which changes to q35
> and try to deploy another new domain from the same prebuilt disk
> image "foo". It may not work now if that disk image doesn't support
> q35. That would be a regression from the user's POV, whether libvirt or
> qemu has changed the default.
>
> Regards,
> Daniel