Re: [Qemu-devel] change x86 default machine type to Q35?

[Paolo Bonzini]

On 05/07/2017 13:58, Thomas Huth wrote:
> On 05.07.2017 13:44, Fam Zheng wrote:
>> On Wed, 07/05 12:04, Daniel P. Berrange wrote:
>>> While you can say people should just add '-M pc' that isn't a nice
>>> user experiance, because it makes the assumption that people actually
>>> understand what caused their breakage. When the incompatibilities
>>> arise the error messages are unlikely to give any hint to users that
>>> the problems are caused by the machine type change. So unless someone
>>> is very familiar with debugging QEMU, they're not going to realize
>>> that '-M pc' will fix their problem.
>>
>> While we change the default, can we start to print a message like 'machine 
>> type
>> ("-M" option) not specified, default to Q35. If you find compatibility 
>> issues,
>> try "-M pc"'?
> 
> Or we could simply remove the default machine type completely for a
> couple of releases? That would force people to update their script with
> "-M pc" ... and once this has been established, we can finally make q35
> the default?

Apart from the user experience issues pointed out by Fam, this would
lead to people using "-M pc" without understanding it, and not being
able to use q35 features.

I think there are two cases:

- if we are worried about command-line users' IDE VMs not booting after
the q35 upgrade (because the PATA adapter becomes SATA/AHCI), we cannot
change the default in QEMU, period.  This would affect at least Windows
VMs; Fedora and derivatives should have both ATA_PIIX and SATA_AHCI as
built-in drivers, I don't know about Debian derivatives and e.g. Gentoo.

- if we are not, and we think q35 is okay for every new VM except
Windows 95 or 2003, we can change the default to "-M q35".  Windows 95
and 2003 users can be asked to add "-M pc".

In the second case, libvirt could still switch from pc to q35 and/or
libosinfo could be changed to suggest q35 instead of pc.

If possible, NICs should keep the same PCI address after upgrade.

Paolo