VirtIO PackedRing Configuration support

This blueprint proposes to expose the LibVirt packed option that allows a guest to negotiate support for the VirtIO packed-ring feature. This blueprint is used to solicit community’s input.

Problem description

VM using a Virtio-net paravirtual network device uses Virtual queues (virtqs) to send and recveive data between the virtio-net driver and the virtual or physical backed. The VirtIO standard originally defined a single type of virtq called split-ring queue. The latest edition of the standard (v1.1) adds a different type of the virtq, called packed-ring queue. A different layout of queue elements allows to increase the performance in both virtual and physical backeds.

Split-ring support is the default option in VirtIO. Backends supporting the packed-ring virtqs advertise this by setting the VIRTIO_F_RING_PACKED feature bit during the feature negotiation. A guest driver then chooses the virtq layout based on what it supports. As both options are identical features wise, and the packed-ring is more efficient, the latter is typically chosen.

Qemu added support for the packed virtqs in v4.2 and LibVirt in v6.3. Qemu and LibVirt supports the packed-ring virtqs via the packed option. However, note that this option does not force the VM to use the packed-ring virtq. It acts as a mask, allowing the backed to advertise the support when set. The driver in the VM is still responsible for choosing the layout of virtqs.

This blueprint proposes to add a Nova flavor extra_spec and Glance image property, that sets the packed option to true on the node. This way all VMs running on the node are allowed to choose the virtq layout based on what is offered by the backed, rather than being froced to use split-ring.

Use Cases

As an operator, I want to benefit from the increase in the virtio-net performance, by using a more efficient virtq structure.

Proposed change

  • Add hw_virtio_packed_ring for image property and hw:virtio_packed_ring for flavor extra specs. Users will control the packed virtqueue feature, and be able to disable it if desired.

    hw_virtio_packed_ring=true|false (default false) hw:virtio_packed_ring=true|false (default false)

  • Provide new compute COMPUTE_NET_VIRTIO_PACKED capablity trait. This trait can be required/forbidden by user. Nova-compute agent will automatically set this trait to the resource provider summary if libvirt version is higher than 6.3

  • This spec will update scheduling process. ALL_REQUEST_FILTERS will be extended with new filter packed_virtqueue_filter. It will update RequestSpec with new trait in case if image property or flavor extra_spec is enabled to avoid migration to the node without packed virtqueue feature support.


Leave as-is, operator will not have additional performance impact.

Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact

VMs using virtio-net will see an increase in performance. The increase can be anywhere between 10/20% (see DPDK Intel Vhost/virtio perf. reports) and 75% (using Napatech SmartNICs).

Other deployer impact


Developer impact


Upgrade impact

  • This spec will update scheduling process. New trait COMPUTE_NET_VIRTIO_PACKED will be set to the resource provider trait list automatically if this feaure is supported on the host.

  • New Functional and Unit tests will be provided.



Primary assignee:

justas_napa on IRC and Gerrit

The feature can be implemented by the Napatech devs and

Feature Liaison

  • Sean Mooney (sean-k-mooney)

Work Items

N/A at this stage.





Documentation Impact

Configuration options reference will require an update.




Release Name


2023.1 Bobcat