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

On 11/07/2017 10:48, Thomas Huth wrote:
On 10.07.2017 19:47, Eduardo Habkost wrote:
(CCing libvir-list)

On Mon, Jul 10, 2017 at 07:45:54PM +0300, Michael S. Tsirkin wrote:
On Mon, Jul 10, 2017 at 10:59:43AM -0300, Eduardo Habkost wrote:
On Mon, Jul 10, 2017 at 10:42:26AM +0300, Marcel Apfelbaum wrote:
On 07/07/2017 21:03, Eduardo Habkost wrote:
On Fri, Jul 07, 2017 at 06:17:57PM +0300, Michael S. Tsirkin wrote:
On Fri, Jul 07, 2017 at 10:39:49AM -0300, Eduardo Habkost wrote:
On Wed, Jul 05, 2017 at 12:32:10PM +0300, Marcel Apfelbaum wrote:

The upper layers should manage the defaults by themselves so
are not supposed to be affected.

But they would be.  libvirt uses the default machine-type from

How about extending the command for supported machines with a
recommended machine type, and teaching libvirt to use that?

I don't think QEMU has enough information to decide if it should
recommend "q35" or "pc".

We don't really need a complicated rule set, we would just recommend q35
by default. Libvirt will try to create the default machine and if fails
for some reason (what would it be?) it can switch to PC.

The advanced logic would be "old systems should use PC", where old
means Windows XP and before and so on. But this logic should appear
in management layers above.

In this case, is there any difference between "changing the
default to q35" and "recommending q35", for libvirt users?


No but libvirt users do not manage e.g. pci slots manually.
They do not use the -cdrom flag.
So changing the default is unlikely to break things for them.

I see.  If this part is really true (can libvirt developers
confirm that?), then the proposed end result makes sense (not
having a default for running QEMU directly, but changing default
to "q35" for people using libvirt).

But I don't see why we would need a new mechanism to make QEMU
recommend a machine-type for libvirt, if libvirt could simply
choose its own default (or maybe also refuse to pick a default,
if libvirt developers decide that's the best solution).

Hi Thomas,

Agreed, it does not make much sense to invent a new mechanism here. I
guess the default should rather be switched in the the tools that create
the XML for libvirt, i.e. virt-install and friends?

Concerning QEMU, could we maybe simply emit a warning a la

  "you did not specify a machine type with the -M option, so you are
   currently running the the 'pc' machine type. Please note that future
   versions of QEMU might use the 'q35' machine type instead. If you
   require the 'pc' machine type for your setting, then please specify
   it with the -M option."

for a couple of releases, so that other people have a chance to update
their scripts, and then finally switch to q35 ?

Sounds like a plan, adding Laine (libvirt) to confirm (or not)
if makes sense.

Is a pity to loose the "QEMU 3.0" release, but is nicer indeed
to let people know in advance.



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