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

[Daniel P . Berrangé]

On Tue, Jun 05, 2018 at 03:44:14PM +0200, Laszlo Ersek wrote:
> On 06/05/18 15:29, 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". 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.
> 
> How about:
> - "create new domain with empty disk" --> i440fx now, q35 later

"empty disk" is not something that can be determined by the
host - libvirt might not even have direct access to the disk
at the time this info would be needed.

> - "create new domain from domain XML and disk image" --> whatever the
>   domain definition dictates
> - "create new domain from disk image and no domain XML" --> assume
>   i440fx forever (with a detailed board / device config that's used for
>   all legacy (definition-less) disk images)



> - convince disk image distributors to provide their domain definitions
>   with their disks (need not be libvirt domain XML, other definitions
>   might work)

Libvirt domain XML is absolutely not suitable - it is a host specific
instantiation of a guest, that is not guaranteed to be portable to
any other host.

Funnily enough though, I just remembered that 10 years ago we invented
another XML format called "virt-image XML", that went along with a
"virt-image" command line tool

  
https://github.com/virt-manager/virt-manager/blob/2e7d477156e9d0f6fb218fa19fc00d6229d33e85/man/virt-image-xml.pod

This was rarely used because the "virt-image" tool itself was rather
broken by design, so we eventually deleted this entirely.

The virt-image XML  description though could be resurrected as it
is largely relevant to the conversation.

> - write converters from those other definition formats to libvirt XML,
>   or QEMU cfg file?

Regards,
Daniel
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