Report CPU features to the placement service

Traits are already supported by the placement service. All the compute node’s capabilities should be reported to placement using the traits API. This spec proposes reporting the cpu features of compute node to placement using traits API.

Problem description

Currently Nova supports scheduling an instance based on the compute node’s CPU features. But it isn’t perfect for now.

  • There is no consistent naming for the CPU features in all the virt drivers. This leads to an inconsistent setup for operators.

  • The CPU features of each compute node are stored in a JSON-BLOB of the compute_nodes DB table. It is therefore not possible to efficiently query for compute nodes having certain CPU features.

  • The flavor extra spec capabilities mix the cpu features and other things together, and it depends on the virt driver.

Use Cases

  • The end user expects a standard and consistent naming for the CPU features regardless of the virt driver used by the cloud.

  • The end user expects a consistent way to request capabilities for the CPU features.

Proposed change

Proposes the compute nodes should report CPU features to placement using the traits API. The CPU features should map to the CPU traits which are defined in the os-traits library.

Then the administrator can define the required traits for each flavor in the extra spec traits:

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

The above commands set two CPU features as required for a flavor. It means that any server booting up with that flavor will be scheduled to the compute node which has the CPU features avx and avx2.


If just keep the old way for the scheduling instance based on the cpu features, there is nothing improvement on the cloud interoperability.

Data model impact


REST API impact

No API change, and the original cpu feature reporting to the DB table compute_nodes will be kept for the /os-hypervisors API backward compatible.

Security impact


Notifications impact


Other end user impact

The ends user should use the extra spec traits in the flavor instead of the extra spec capabilities:cpu_info:features to define required CPU features.

Performance Impact


Other deployer impact

It will be possible to deprecate and remove the ComputeCapabilitiesFilter once all virt drivers are reporting CPU features consistently using traits.

Developer impact


Upgrade impact



Each virt-driver should attach CPU traits to the compute node’s resource provider in the method update_provider_tree().


The configure option CONF.libvirt.cpu_mode and CONF.libvirt.cpu_model can change the CPU features exposed to the guest. The libvirt virt-driver should only return the CPU features which are available to the guest.

The reported CPU features are based on the guest CPU model. The CPU model will be determined by the table as below:

| CONF.libvirt.cpu_mode | CONF.libvirt.cpu_model | Guest CPU model  |
| none                  | N/A                    | qemu64           |
| host-model            | N/A                    | host cpu model   |
| host-passthrough      | N/A                    | host cpu model   |
| custom                | custom cpu model       | custom cpu model |

For the libvirt virt driver, the virConnectBaselineCPU libvirt API call to query for CPU model and translate the model into CPU features. Other virt drivers can use similar calls in their virt APIs to determine CPU features.

There is no same configuration for other virt drivers. For other virt drivers, they will report CPU traits same as the current way.


Primary assignee:

Alex Xu <>

Other contributors:

Lei Zhang <>

Work Items

  • Implement an interface which returns a list of traits for the CPU features in each virt driver.

  • Implement report CPU traits to the compute node resource provider in the update_provider_tree() method.




The related unit-tests and functional tests are required.

Documentation Impact

In the administrator guideline to explain how to use traits to define the required CPU features.





Release Name