Libvirt: support vDPA based networking

Over the years a number of different technologies have been developed to offload networking and other function to external processes in order to accelerate QEMU instance performance. In kernel 5.7+ a new virtual bus know as vDPA (vHost data path acceleration) was introduced to provide a vendor neutral way to accelerate standard virtio device using software or hardware accelerator implementations. vDPA support was introduced in Libvirt 6.9.0 to leverage the vDPA capabilities introduced in QEMU 5.1+. This blueprint tracks enhancing nova to leverage these new capabilities for offloading to hardware-based smart NICs via hardware offloaded OVS.

Problem description

Current hardware offloaded networking solutions require vendor specific drivers in the guest to function. The vDPA bus allows an abstraction layer to exist between the accelerator and the VM without the CPU overhead of traditional software vHost implementations. vDPA-enabled vSwitch offloads allow the guest to use standard virtio drivers instead of a vendor specific driver.


While vDPA is expected to support live migration in the future, QEMU does not currently support live migration with vDPA devices. One of the main advantages of vDPA based networking over SR-IOV is the ability to abstract the device state from the VM, allowing transparent live migration via a software fallback. Until that fallback is implemented in QEMU, live migration will be blocked at the API layer via a HTTP 409 (Conflict) error response so that we can enable it without a new micro-version.

As Open vSwitch is currently the only control plane capable of managing vDPA devices, and since that requires hardware offloads to function, this spec will focus on enabling vDPA networking exclusively with hardware-offloaded OVS. In a future release this functionality can be extended to other vSwitch implementations such as VPP or linux bridge if they become vDPA enabled.

Use Cases

As an operator, I want to offer hardware-accelerated networking without requiring tenants to install vendor-specific drivers in their guests.

As an operator, I want to leverage hardware-accelerated networking while maintaining the ability to have transparent live migration.


Transparent live migration will not initially be supported and will be enabled only after it is supported officially in a future QEMU release.

Proposed change

  • A new vnic-type vdpa has been introduced to neutron to request vDPA offloaded networking.

  • The class will be extended to define the new vdpa vnic-type constant.

  • The libvirt driver will be extended to generate the vDPA interface XML

  • The PCI tracker will be extended with a new device-type, VDPA. While the existing whitelist mechanism is unchanged, if a device is whitelisted and is bound to a vDPA driver, it will be inventoried as VDPA. In the libvirt driver this will be done by extending the private _get_pci_passthrough_devices and _get_device_type functions to detect if a VF is a parent of a vDPA nodedev. These functions are called by get_available_resources in the resource tracker to generate the resources dictionary consumed by _setup_pci_tracker at the startup of the compute agent in _init_compute_node.


The vDPA device type is required to ensure that the VF associated with vDPA devices cannot be allocated to VMs via PCI alias or standard neutron SR-IOV support. VFs associated with vDPA devices cannot be managed using standard kernel control plane command such as ip. As a result, allocating them to an interface managed by the sriov-nic-agent or via alias-based PCI passthrough is not valid. This will also provide automatic NUMA affinity and a path to eventurally report vDPA devices in placement as part of generic PCI device tracking in placement in the future.


We could delegate vDPA support to cyborg. This would still require the libvirt changes and neutron changes while also complicating the deployment. Since vDPA-based NICs are fixed function NICs there is not really any advantage to this approach that justifies the added complexity of inter-service interaction.

We could use the resources table added for vPMEM devices to track the devices in the DB instead of the PCI tracker. This would complicate the code paths as we would not be able to share any of the PCI NUMA affinity code that already exists.

We could support live migration by treating vDPA devices as if they are direct mode SR-IOV interface, meaning nova would hot unplug and plug the interface during the migration. In the future this could be replaced with transparent live migration if the QEMU version on both hosts is new enough. Since we don’t know when that will be, this option is deferred until a future release to reduce complexity.

A new workaround config option could be added enable_virtual_vdpa_devices=True|False (default: False). When set to True it would allow a virtual vDPA devices such as the vdpa_sim devices to be tracked and used. Virtual PCI devices do not have a VF or PCI device associated with them, so setting this value would result in the no-op os-vif driver being used and a sentinel value being used to track the device in the PCI tracker. This would allow testing without real vDPA hardware in CI and is not intended for production use. This was declared out of scope to avoid adding a config option and code that is purely used for testing. Functional tests will be used instead to ensure adequate code coverage.

A new standard trait HW_NIC_VDPA could be reported by the libvirt driver on hosts with vDPA devices, and the required version of QEMU and libvirt. This would be used in combination with a new placement prefilter to append a required trait request to the unnamed group if any VM interface has vnic-type vdpa. This will not be done as it will not be required when PCI devices are tracked in placement. Since standard traits cannot be removed, no new trait will be added and the PCI passthrough filter will instead be used to filter host on device type.

Data model impact

The allowed values of the vnic-type in the class will be extended. The allowed values of the device_type column in the pci_devices table will be extended.

Optionally, the VIFMigrateData or LibvirtLiveMigrateData object can be extended to denote the destination host support transparent vDPA live migration. This is optional as currently it would always be false until a QEMU and libvirt version are released that support this feature.

REST API impact

The neutron port-binding extension vnic-types has been extended to add vnic-type vdpa. No nova API changes are required.

Security impact


Notifications impact


Other end user impact

vDPA ports will work like SR-IOV ports from an end user perspective, however the device model presented to the guest will be a virtio NIC and live migration will initially be blocked until supported by QEMU.

Performance Impact

The performance will be the same as SR-IOV in terms of dataplane performance and nova scheduling or VM creation a.k.a. None.

Other deployer impact

vDPA requires a very new kernel and very new versions of QEMU and libvirt to use. Initial support for vDPA was added in kernel 5.7, QEMU 5.1 and libvirt 6.9.0.

The operator will need to ensure all dependencies are present to use this feature. Intel NIC support is present in 5.7 at the time of writing, no NIC that support vDPA is available on the market from Intel. The first publicly available NICs with vDPA capabilities are the Mellanox/Nvidia ConnectX-6 DX/LX NICs, which are only enabled in kernel 5.9

Developer impact


Upgrade impact




Primary assignee:


Other contributors:


Feature Liaison


Work Items

  • Update libvirt driver

  • Add prefilter

  • Add docs

  • Update tests


  • libvirt 6.9.0+

  • QEMU 5.1+

  • Linux 5.7+


This will be tested primarily via unit and functional tests, however a Tempest job using the vDPA sim module may be created if it proves practical to do so. The main challenges to this are creating a stable testing environment with the required dependencies. Fedora rawhide has all the required dependencies but ships with python 3.9. OpenStack currently does not work properly under python 3.9

Alternative test environments such as Ubuntu 20.04 do not provide new enough kernel by default or do not ship the required libvirt and QEMU versions. Compilation from source is an option but we may or may not want to do that in the upstream CI.

Documentation Impact

The existing admin networking document will be extended to introduce vDPA and describe the requirement for use.


The nova-neutron PTG discussion on this topic can be found on line 186 here:

An introduction to this topic and is available as a blog at and other blogs on the topic covering the evolution and current state are also available



Release Name





Updated to reflect changes to HTTP error codes