hw_machine_type - Configuring and updating QEMU instance machine types

New in version 12.0.0: (Liberty)

Changed in version 23.0.0: (Wallaby)

The libvirt driver now records the machine type of an instance at start up allowing the [libvirt]hw_machine_type configurable to change over time without impacting existing instances.

Added nova-manage commands to control the machine_type of an instance.

Changed in version 25.0.0: (Yoga)

Added nova-manage commands to set the image properties of an instance.


The following only applies to environments using libvirt compute hosts.


QEMU’s machine type concept can be thought of as a virtual chipset that provides certain default devices (e.g. PCIe graphics card, Ethernet controller, SATA controller, etc). QEMU supports two main variants of “machine type” for x86 hosts: (a) pc, which corresponds to Intel’s I440FX chipset (released in 1996) and (b) q35, which corresponds to Intel’s 82Q35 chipset (released in 2007). For AArch64 hosts, the machine type is called: virt.

The pc machine type is considered legacy, and does not support many modern features. Although at this time of writing, upstream QEMU has not reached an agreement to remove new versioned variants of the pc machine type, some long-term stable Linux distributions (CentOS, RHEL, possibly others) are moving to support q35 only.


For end users the machine type of an instance is controlled by the selection of an image with the hw_machine_type image metadata property set.

$ openstack image set --property hw_machine_type=q35 $IMAGE

The libvirt virt driver supports the configuration of a per compute host default machine type via the libvirt.hw_machine_type option. Providing a default machine type per host architecture to be used when no corresponding hw_machine_type image property is provided for the instance.

When this option is not defined the libvirt driver relies on the following hardcoded dictionary of default machine types per architecture:

default_mtypes = {
    obj_fields.Architecture.ARMV7: "virt",
    obj_fields.Architecture.AARCH64: "virt",
    obj_fields.Architecture.S390: "s390-ccw-virtio",
    obj_fields.Architecture.S390X: "s390-ccw-virtio",
    obj_fields.Architecture.I686: "pc",
    obj_fields.Architecture.X86_64: "pc",


Prior to the Wallaby (23.0.0) release the libvirt.hw_machine_type option had to remain static once set for the lifetime of a deployment. This was due to the machine type of instances without a hw_machine_type image property using the newly configured machine types after a hard reboot or migration This could in turn break the internal ABI of the instance when changing between underlying machine types such as pc to q35.

From the Wallaby (23.0.0) release it is now possible to change the libvirt.hw_machine_type config once all instances have a machine type recorded within the system metadata of the instance.

To allow this the libvirt driver will now attempt to record the machine type for any instance that doesn’t already have it recorded during start up of the compute service or initial spawn of an instance. This should ensure a machine type is recorded for all instances after an upgrade to Wallaby that are not in a SHELVED_OFFLOADED state.

To record a machine type for instances in a SHELVED_OFFLOADED state after an upgrade to Wallaby a new nova-manage command has been introduced to initially record the machine type of an instance.

$ nova-manage libvirt update_machine_type $instance $machine_type

This command can also be used later to update the specific machine type used by the instance. An additional nova-manage command is also available to fetch the machine type of a specific instance:

$ nova-manage libvirt get_machine_type $instance

To confirm that all instances within an environment or a specific cell have had a machine type recorded another nova-manage command can be used:

$ nova-manage libvirt list_unset_machine_type

The logic behind this command is also used by a new nova-status upgrade check that will fail with a warning when instances without a machine type set exist in an environment.

$ nova-status upgrade check

Once it has been verified that all instances within the environment or specific cell have had a machine type recorded then the libvirt.hw_machine_type can be updated without impacting existing instances.

Device bus and model image properties

New in version 25.0.0: (Yoga)

Device bus and model types defined as image properties associated with an instance are always used when launching instances with the libvirt driver. Support for each device bus and model is dependent on the machine type used and version of QEMU available on the underlying compute host. As such, any changes to the machine type of an instance or version of QEMU on a host might suddenly invalidate the stored device bus or model image properties.

It is now possible to change the stored image properties of an instance without having to rebuild the instance.

To show the stored image properties of an instance:

$ nova-manage image_property show $instance_uuid $property

To update the stored image properties of an instance:

$ nova-manage image_property set \
    $instance_uuid --property $property_name=$property_value