CPU models

Nova allows you to control the guest CPU model that is exposed to instances. Use cases include:

  • To maximize performance of instances by exposing new host CPU features to the guest

  • To ensure a consistent default behavior across all machines, removing reliance on system defaults.


The functionality described below is currently only supported by the libvirt driver.

CPU modes

In libvirt, the CPU is specified by providing a base CPU model name (which is a shorthand for a set of feature flags), a set of additional feature flags, and the topology (sockets/cores/threads). The libvirt KVM driver provides a number of standard CPU model names. These models are defined in /usr/share/libvirt/cpu_map/*.xml. You can inspect these files to determine which models are supported by your local installation.

Two Compute configuration options in the libvirt group of nova.conf define which type of CPU model is exposed to the hypervisor when using KVM: libvirt.cpu_mode and libvirt.cpu_models.

The libvirt.cpu_mode option can take one of the following values: none, host-passthrough, host-model, and custom.

See Effective Virtual CPU configuration in Nova for a recorded presentation about this topic.

Host model

If cpu_mode=host-model, the CPU model in /usr/share/libvirt/cpu_map/*.xml that most closely matches the host, and requests additional CPU flags to complete the match. This configuration provides the maximum functionality and performance and maintains good reliability.

With regard to enabling and facilitating live migration between compute nodes, you should assess whether host-model is suitable for your compute architecture. In general, using host-model is a safe choice if your compute node CPUs are largely identical. However, if your compute nodes span multiple processor generations, you may be better advised to select a custom CPU model.

The host-model CPU model is the default for the KVM & QEMU hypervisors (libvirt.virt_type=``kvm``/qemu)

Host passthrough

If cpu_mode=host-passthrough, libvirt tells KVM to pass through the host CPU with no modifications. In comparison to host-model which simply matches feature flags, host-passthrough ensures every last detail of the host CPU is matched. This gives the best performance, and can be important to some apps which check low level CPU details, but it comes at a cost with respect to migration.

In host-passthrough mode, the guest can only be live-migrated to a target host that matches the source host extremely closely. This definitely includes the physical CPU model and running microcode, and may even include the running kernel. Use this mode only if:

  • Your compute nodes have a very large degree of homogeneity (i.e. substantially all of your compute nodes use the exact same CPU generation and model), and you make sure to only live-migrate between hosts with exactly matching kernel versions, or

  • You decide, for some reason and against established best practices, that your compute infrastructure should not support any live migration at all.


If cpu_mode=custom, you can explicitly specify an ordered list of supported named models using the libvirt.cpu_models configuration option. It is expected that the list is ordered so that the more common and less advanced CPU models are listed earlier.

In selecting the custom mode, along with a libvirt.cpu_models that matches the oldest of your compute node CPUs, you can ensure that live migration between compute nodes will always be possible. However, you should ensure that the libvirt.cpu_models you select passes the correct CPU feature flags to the guest.

If you need to further tweak your CPU feature flags in the custom mode, see CPU feature flags.


If libvirt.cpu_models is configured, the CPU models in the list needs to be compatible with the host CPU. Also, if libvirt.cpu_model_extra_flags is configured, all flags needs to be compatible with the host CPU. If incompatible CPU models or flags are specified, nova service will raise an error and fail to start.


If cpu_mode=none, libvirt does not specify a CPU model. Instead, the hypervisor chooses the default model.

The none CPU model is the default for all non-KVM.QEMU hypervisors. (libvirt.virt_type!=``kvm``/qemu)

CPU feature flags

New in version 18.0.0: (Rocky)

Regardless of your configured libvirt.cpu_mode, it is also possible to selectively enable additional feature flags. This can be accomplished using the libvirt.cpu_model_extra_flags config option. For example, suppose you have configured a custom CPU model of IvyBridge, which normally does not enable the pcid feature flag, but you do want to pass pcid into your guest instances. In this case, you could configure the following in nova.conf to enable this flag.

cpu_mode = custom
cpu_models = IvyBridge
cpu_model_extra_flags = pcid

An end user can also specify required CPU features through traits. When specified, the libvirt driver will select the first CPU model in the libvirt.cpu_models list that can provide the requested feature traits. If no CPU feature traits are specified then the instance will be configured with the first CPU model in the list.

Consider the following nova.conf:

cpu_mode = custom
cpu_models = Penryn,IvyBridge,Haswell,Broadwell,Skylake-Client

These different CPU models support different feature flags and are correctly configured in order of oldest (and therefore most widely supported) to newest. If the user explicitly required the avx and avx2 CPU features, the latter of which is only found of Haswell-generation processors or newer, then they could request them using the trait{group}:HW_CPU_X86_AVX and trait{group}:HW_CPU_X86_AVX2 flavor extra specs. For example:

$ openstack flavor set $FLAVOR \
    --property trait:HW_CPU_X86_AVX=required \
    --property trait:HW_CPU_X86_AVX2=required

As Haswell is the first CPU model supporting both of these CPU features, the instance would be configured with this model.