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

[Marcel Apfelbaum <marcel.apfelbaum@gmail.com>]

On 06/04/2018 07:56 PM, Daniel P. Berrangé wrote:
> On Mon, Jun 04, 2018 at 07:48:51PM +0300, Michael S. Tsirkin wrote:
>> On Mon, Jun 04, 2018 at 02:01:29PM +0100, Daniel P. Berrangé wrote:
>>> On Mon, Jun 04, 2018 at 09:54:15AM -0300, Eduardo Habkost wrote:
>>>> On Mon, Jun 04, 2018 at 04:38:22AM +0300, Michael S. Tsirkin wrote:
>>>>> On Sun, Jun 03, 2018 at 12:27:49PM +0300, Marcel Apfelbaum wrote:
>>>>>> Moving to QEMU 3.0 seems like a good opportunity for such a change.
>>>>>>
>>>>>> I440FX is really old and does not support modern features like IOMMU.
>>>>>> Q35's SATA emulation is faster than pc's IDE, native PCI express hotplug
>>>>>> is cleaner than ACPI based one and so on...
>>>>>>
>>>>>> Also the libvirt guys added very good support for the Q35 machine (thanks!).
>>>>>>
>>>>>> Management software should always specify the machine type and for the
>>>>>> current setups, adding '-machine pc' to the command line is not such a
>>>>>> big deal.
>>>>>>
>>>>>> In time the pc machine will fade out and we will probably stop adding
>>>>>> new versions at some point.
>>>>>>
>>>>>> Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
>>>>> For command line users, I think changing the default isn't nice.
>>>>>
>>>>> Yes it's easy to add -machine pc but there's no documentation
>>>>> that tells you to do so. Add to that shortcuts like -cdrom
>>>>> stop working, hotplug needs extra bridges to work, and one
>>>>> can see that while management tool users benefit from q35,
>>>>> command line users will suffer.
>>>>>
>>>>> Can't we add a tag for management without changing the command line
>>>>> default? How about "management-default"? "recommended"? "latest"?
>>>> We could add new aliases if they are useful for management
>>>> software, but we would need a well-defined use case and set of
>>>> requirements+expectations for the new alias.
>>> I'm not convinced by the idea of adding a distinct default "for mgmt". All
>>> the problems described wrt 'q35' vs 'pc' apply equally to management apps
>>> as they do to humans. It just happens that one common mgmt layer (libvirt)
>>> knows how to handle some of the complexity of q35. Other mgmt apps though
>>> are just as likely to be hurt by the change as humans are. So effectively
>>> the proposed "for mgmt" is actually  "for libvirt >= some version", which
>>> feels like a layering violation to me.
>> Is libvirt happy to just hard-code q35 for now then?
> I'm pretty wary of doing that, as I feel 'pc' has broader OS compatibility
> than 'q35', so we'd be likely to cause breakage for users.
>
> IMHO, defaults are something better expressed in libosinfo, so if there's
> guests where we think q35 is a better choice, we can record it there and
> leave everything else on pc to avoid risk of breakage.

The only info we need to pass properly to management systems is:
"Use q35 unless your guests are really old".

I agree the exiting systems should not be touched.

Thanks,
Marcel

> Regards,
> Daniel