Select CPU model from a list of CPU models

Problem description

In the libvirt driver, currently we use cpu_model in nova.conf (when cpu_mode is set to custom) to specify the CPU model the instance should use on this host. This could have implications on availability of compute nodes for live migration.

  • If the instance lands on a compute node which uses an “advanced” CPU model, then it may only live-migrate to a few of the cluster’s compute nodes or fail to live-migrate.

  • The admin can configure all compute nodes use the same CPU model. But some users may request “advanced” CPU flags for some special application (such as video edit and scientific compute).

Use Cases

As an admin, I would like to live-migrate instances among all compute nodes even if there are different CPU models in the the cluster.

As a user, I would like to boot an instance on a host supporting specific CPU features and for the instance to be live-migratable to as many other hosts as possible that also support the instance.

Proposed change

Rename the cpu_model config attribute with cpu_models, which is an ordered list of CPU models the host supports. It is expected that the list is ordered so that the more common and less advanced CPU models are listed earlier. The reported CPU feature traits will be the union of features of all the CPU models. Mark cpu_model is deprecated, so existing confs will continue to work, but will log a warning.

Note that this is not add a new config attribute, this is rename exist config attribute and extend it from singular to plural variants. Mark cpumodel as deprecated is to maintain capatibility with the old confs. Otherwise, supporting both the singular and plural variants at the same time (or even after we deprecate cpu_model) could lead to confusion and avoidable typos. Avoid them.

End users specify CPU features they require through traits [1]. If the cpu_mode is set to custom, the libvirt driver will select the first CPU model in the cpu_models (combined with cpu_model_extra_flags if it is specified) that can provide the required feature traits. This would make it more likely that the instance could be live-migrated later on. If no CPU feature traits are specified then the instance will be configured with the first CPU model in the list.

For example, if the end user specifies CPU features avx and avx2 as follows:

openstack flavor set 1 --property trait:HW_CPU_X86_AVX=required --property trait:HW_CPU_X86_AVX2=required

and cpu_models is configured like this:

cpu_mode = custom
cpu_models = SandyBridge,IvyBridge,Haswell,Broadwell

then Haswell, the first CPU model supporting both avx and avx2, will be chosen by libvirt.

If cpu_model_extra_flags or cpu_models specified, it should be checked against each configured items to make sure they are compatible with host CPU. Any incompatibility should prevent the compute service from starting and force the user to correct the configuration.

A few related points:

  • If both cpu_models and cpu_model are set, cpu_model will be ignored.

  • Typically, data centers only have a handful of CPU generations deployed, so the cpu_models is expected to contain not as many CPUs as shown in the contrived example earlier.

  • The value in the option cpu_models will be made case-insensitive.



Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact


Other deployer impact

Add some information in the config option help text indicating that the operator should be careful to only specify models which can be fully supported in hardware. If they specify models with CPU features that are emulated by QEMU it could result in performance degradation.

Developer impact


Upgrade impact





Work Items

  • Conf: Rename [libvirt]cpu_model to [libvirt]cpu_models.

  • Virt driver changes.

  • Add/modify unit tests.




Add unit tests.

Documentation Impact

Update release note for introducing [libvirt]cpu_models.


[2] Stein iteration of this spec:

[3] The work in progresss spec to add more “hypervisor-literate” CPU

APIs to Nova – (“CPU selection with hypervisor consideration”)



Release Name