Add Virtual IOMMU device support for libvirt driver

The spec adds support to expose a virtual IO memory mapping unit (vIOMMU) with libvirt driver.

Problem description

Currently it is possible to use libvirt to expose vIOMMU to a guest when using the x86 Q35 or ARM virt machine types. On some platfroms such as AArch64 an vIOMMU is required to fully support PCI passthough and in general it can enable use of vfio-pci in guests that require it. Nova does not currently expose vIOMMU functionality to operators or users.

Use Cases

  • As an operator deploying nova on aarch64, I would like to be able to leverage PCI passthrough to support assigning accelerators and other PCIe devices to my guests.

  • As an operator, I would like to enable my end users to use dpdk in their vms

  • As a vnf vendor, that delivers applications that leverage accelerators that require an iommu I would like to express that as an attribute of the image.

  • As an operator, I would like to nova to expose vIOMMU capability on a host that supports it and automatically place vms that requires it on appropriate hosts.

Proposed change

  • This spec proposes adding new guest configs for IOMMU (LibvirtConfigGuestIOMMU) and APIC feature (LibvirtConfigGuestFeatureIOAPIC).

  • Add following attribute to image property and extra_specs:

    • hw_viommu_model (for image property) and hw:viommu_model (for extra_specs): Support values none|intel|smmuv3|virtio|auto. Default to none. auto will select virtio if Libvirt supports it, else intel on X86 and smmuv3 on AArch64.

    above attribute is on of options for LibvirtConfigGuestIOMMU, More information for them can be found in libvirt format domain.

  • Add IOMMU config when generating guest config. And enable IOAPIC within.

  • Add hw_locked_memory for image property and hw:locked_memory for extra specs. This will make sure locked element is present in the memoryBacking, but only allow it if you have also set hw:mem_page_size, so we can ensure that the scheduler can actually account for this correctly and prevent out of memory events. Here is a reference to related issue MEMLOCK_RLIMIT. Locked memory not only disables memory over subscription but it also prevent the kernel form swapping the memory. Enable this will disable the RLIMITs for the VM in cases where you have a large number of passed through devices. When assigning multiple devices to the same VM. The issue is that with a guest IOMMU, each assigned device has a separate address space that is initially configured to map the full address space of the VM and each vfio container for each device is accounted separately. Libvirt will only set the locked memory limit to a value sufficient for locking the memory once, whereas in this configuration we’re locking it once per assigned device. Without a guest IOMMU, all devices run in the same address space and therefore the same container, and we only account the memory once for any number of devices (with hw:mem_page_size set to any value this will enable the NUMA toplogy fitler to schdule based on the fact the memory can’t be over commited).

  • For aw_bits attribute in LibvirtConfigGuestIOMMU: This attribute can used to set the address width to allow mapping larger iova addresses in the guest. Since 6.5.0 (QEMU/KVM only). As Qemu current supported values are 39 and 48, I propose we set this to larger width (48) by default and will not exposed to end user.

  • For eim attribute in LibvirtConfigGuestIOMMU: this will not exposed to end user, but will directly enabled if machine type is Q35. Side Note: eim(Extended Interrupt Mode) attribute (with possible values on and off) can be used to configure Extended Interrupt Mode. A q35 domain with split I/O APIC (as described in hypervisor features), and both interrupt remapping and EIM turned on for the IOMMU, will be able to use more than 255 vCPUs. Since 3.4.0 (QEMU/KVM only).

  • Provide iommu model trait for each viommu model.

  • Add hw_viommu_model to request_filter, this will extend the transform_image_metadata prefilter to select host with the correct model.

  • Provide new compute COMPUTE_IOMMU_MODEL_* capablity trait for each model it supports in driver.



Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact

Enable vIOMMU might introduce significant performance overhead. You can see performance comparision table from AMD vIOMMU session on KVM Forum 2021. For above reason, vIOMMU should only be enable for workflow that require it.

Other deployer impact

Operators will see new extra spec options and image properties.

Developer impact


Upgrade impact




Primary assignee:


Other contributors:


Feature Liaison

Feature liaison:


Work Items




  • Unit test for in patch.

  • We can work on more advance test against real environment. Not that needed for this patch IMO but we still should provide certain level of examine for extra guarantee.

Documentation Impact

  • New docs for new guest options in extra_specs and image properties documentation.