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

[Laszlo Ersek]

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

(I'm with Markus:
<http://mid.mail-archive.com/address@hidden>. Not
specifically about OVF, but disk image providers need to start exposing
their QEMU configs. And I think they should do that in whatever format
their management stack supports.)

Just my two cents, feel free to ignore.

Thanks
Laszlo