Introduction

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.

Configure

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",
}

Update

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

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