Support interface attach with QoS ports

Since microversion 2.72 nova supports creating servers with neutron ports having resource request. In the recent releases support for the lifecycle operations with such ports have also been added. The next step is to support attaching QoS ports to running instances.

Problem description

Nova does not support attaching ports to a running instance if the port has resource request. Such interface attach is rejected since Neutron added support for port resource request in Stein.

Use Cases

As an end user, I would like to add a new interface to my server that has QoS minimum bandwidth rules and I want that such operation only succeeds if the requested bandwidth can be guaranteed.

Proposed change

The attach_interface RPC handler in the ComputeManager needs to be extended with the following logic:

  • Gather the port resource request from Neutron

  • Call placement allocation_candidates API based on the port resource request, but restrict the query with an in_tree filter to the current RP tree.

  • Select the first candidate and note the resource provider mapping of the candidate

  • Update the instance allocation based on the selected candidate

  • Update the InstancePciRequest of the port, if any, with the interface name of the parent physical device based on the device resource provider the port allocates from

  • Do the PCI claim, if any, as today

  • Pass the resource provider mapping to allocate_for_instance call candidate

A couple of new error cases needs to be handled. The attach_interface compute RPC call is synchronous so the error cases could lead to HTTP error codes:

  • Placement returns no allocation candidates then respond with HTTP 400 similarly to NoValidHost

  • Updating the instance allocation fails due to resource conflict then retry with another candidate. If we run out of candidates then respond with HTTP 400

  • Updating the instance allocation fails due to generation conflict then reload allocations from placement and retry the update. If it still fails after 3 retries then respond with HTTP 409

  • Updating the InstancePciRequest with parent interface name fails the respond with HTTP 400

All the error cases keep the server in ACTIVE state and record a failed instance action. Also none of these are introducing new HTTP response code for this API so no new microversion is needed.



Data model impact


REST API impact

POST /servers/{server_id}/os-interface with a Neutron port having resource request will be accepted. This is a similar change to supporting move operations with QoS ports and that was done without bumping a microversion. So this change will also be made without a microversion bump.

Security impact


Notifications impact


Other end user impact


Performance Impact

An extra placement allocation candidate query needs to be made to select which physical device can accommodate the additional resource request on the host and then the instance allocation needs to be updated in placement based on the selected candidate. These queries will only be run if the port has resource request and the allocation candidate query will be restricted to the host the instance is currently running on so the overall performance impact is limited.

Other deployer impact


Developer impact


Upgrade impact

The main implementation of this feature will be in the ComputeManager in the nova-compute service. So the compute service version needs to be bumped. Currently, the API rejects such attach request. The related check in the API needs to be replaced with a service level check to ensure that the attach is only accepted if the compute service hosting the VM is new enough to support the request.



Primary assignee:


Feature Liaison

Feature liaison:


Work Items

See them in the Proposed change section




Unit and functional testing will be provided for both normal and PCI device backed interfaces. Tempest tests will be provided for normal ports only due to the CI system limitation regarding SRIOV capable network devices.

Documentation Impact

The API guide Using ports with resource request will be updated accordingly. Also the Limitations section of the neutron admin guide Quality of Service Guaranteed Minimum Bandwidth needs to be updated.




Release Name