[openstack-dev] [nova] A question about libvirt cpu mode=none configuration

Daniel P. Berrange berrange at redhat.com
Fri Mar 13 13:50:15 UTC 2015

On Fri, Mar 13, 2015 at 08:42:25AM -0400, Steve Gordon wrote:
> ----- Original Message -----
> > From: "park" <jianlonghei at gmail.com>
> > To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org>
> > 
> > hello Nova
> > 
> > By default, nova libvirt driver configuration for guest cpu as "cpu_mode
> > = none",
> > you could add cpu features by changing flavor section as below, there
> > will NOT
> > be any issues for this cmd.
> > 
> > $nova flavor-key m1.small set "capabilities:cpu_info:features"="<in> ***"
> > but, nova will ignore the cpu features after the guest boot up
> > successfully, which
> > means the feature does NOT take effect(not be written into the xml for
> > the guest).
> > 
> > And there is no message telling the users that the features does NOT
> > take effect...
> > this may make the users confused...
> This issue is more general than the cpu_mode=none case, in that the
> capabilities filter is filtering *hosts* based on their CPU features.
> As you have discovered, whether they are actually exposing those
> features to guests in their current configuration is not taken into
> account (that is, cpu_mode/cpu_model settings aren't considered at
> all). Ideally they would be, but I'm not sure this is trivial.

If you set  'cpu_mode=host-model' or 'cpu_mode=host-passthrough'
then it will take effect, as the guest will see the full CPU model
of the host that is picked. IMHO the capabilities:cpu_info:features
filter only makes sense when using those two cpu modes. If you
left the default cpu_mode=None or set cpu_mode=custom, then this
capabilities feature is meaningless from a conceptual POV. So the
fact that it has no effect on the guest CPU is not a bug it is
by design.

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