[libvirt] Modern CPU models cannot be used with libvirt

[Anthony Liguori]

On 03/09/2012 03:04 PM, Daniel P. Berrange wrote:
> On Fri, Mar 09, 2012 at 05:56:52PM -0300, Eduardo Habkost wrote:
>> Resurrecting an old thread:
>>
>> I didn't see any clear conclusion in this thread (this is why I am
>> resurrecting it), except that many were arguing that libvirt should
>> simply copy and/or generate the CPU model definitions from Qemu. I
>> really don't think it's reasonable to expect that.
>>
>> On Thu, Dec 15, 2011 at 03:54:15PM +0100, Jiri Denemark wrote:
>>> Hi,
>>>
>>> Recently I realized that all modern CPU models defined in
>>> /etc/qemu/target-x86_64.conf are useless when qemu is used through libvirt.
>>> That's because we start qemu with -nodefconfig which results in qemu ignoring
>>> that file with CPU model definitions. We have a very good reason for using
>>> -nodefconfig because we need to control the ABI presented to a guest OS and we
>>> don't want any configuration file that can contain lots of things including
>>> device definitions to be read by qemu. However, we would really like the new
>>> CPU models to be understood by qemu even if used through libvirt. What would
>>> be the best way to solve this?
>>>
>>> I suspect this could have been already discussed in the past but obviously a
>>> workable solution was either not found or just not implemented.
>>
>> So, our problem today is basically:
>>
>> A) libvirt uses -nodefconfig;
>> B) -nodefconfig makes Qemu not load the config file containing the CPU
>>     model definitions; and
>> C) libvirt expects the full CPU model list from Qemu to be available.
>
> I could have sworn we had this discussion a year ago or so, and had decided
> that the default CPU models would be in something like /usr/share/qemu/cpu-x86_64.conf
> and loaded regardless of the -nodefconfig setting. /etc/qemu/target-x86_64.conf
> would be solely for end user configuration changes, not for QEMU builtin
> defaults.
>
> But looking at the code in QEMU, it doesn't seem we ever implemented this ?

I don't remember that discussion and really don't think I agree with the conclusion.

If libvirt wants to define CPU models on their own, they can.  If libvirt wants 
to use the user's definitions, don't use -nodefconfig.

CPU models aren't a QEMU concept.  The reason it's in the configuration file is 
to allow a user to add their own as they see fit.  There is no right model 
names. It's strictly a policy.

Regards,

Anthony Liguori

>
> Daniel