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

[Daniel P . Berrangé]

On Tue, Jun 05, 2018 at 06:56:09PM +0300, Marcel Apfelbaum wrote:
> 
> 
> 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

NB there's no reliable distinction of "new" from "old" guest install
from libvirt's POV. When we receive XML from an application we've
no way of knowing if this is a brand new guest about to be installed
or a previously installed guest.

Regards,
Daniel
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