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

[Michael S. Tsirkin]

On Tue, Jun 05, 2018 at 02:51:52PM +0100, Daniel P. Berrangé wrote:
> 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.

Oh, that might be a nice thing to put into qcow2 meta-data.
(Hopefully in a format that's easier to parse and write than xml).


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