Virtual CPU

MOSK provides the capability to configure virtual CPU types for OpenStack instances through the OpenStackDeployment custom resource. This feature enables cloud user to tailor performance and resource allocation within their OpenStack environment to meet specific workload demands effectively.

Parameter

spec:features:nova:vcpu_type

Usage

Configures the type of virtual CPU that Nova will use when creating instances.

The list of supported CPU models include host-model (default), host-passthrough, and custom models.

The host-model CPU model

The host-model CPU model (default) mimics the host CPU and provides for decent performance, good security, and moderate compatibility with live migrations.

With this mode, libvirt finds an available predefined CPU model that best matches the host CPU, and then explicitly adds the missing CPU feature flags to closely match the host CPU features. To mitigate known security flaws, libvirt automatically adds critical CPU flags, supported by installed libvirt, QEMU, kernel, and CPU microcode versions.

This is a safe choice if your OpenStack compute node CPUs are of the same generation. If your OpenStack compute node CPUs are sufficiently different, for example, span multiple CPU generations, Mirantis strongly recommends setting explicit CPU models supported by all of your OpenStack compute node CPUs or organizing your OpenStack compute nodes into host aggregates and availability zones that have largely identical CPUs.

Note

The host-model model does not guarantee two-way live migrations between nodes.

When migrating instances, the libvirt domain XML is first copied as is to the destination OpenStack compute node. Once the instance is hard rebooted or shut down and started again, the domain XML will be re-generated. If versions of libvirt, kernel, CPU microcode, or BIOS firmware differ from what they were on the source compute node the instance was started before, libvirt may pick up additional CPU feature flags, making it impossible to live-migrate back to the original compute node.

The host-passthrough CPU model

The host-passthrough CPU model provides maximum performance, especially when nested virtualization is required or if live migration support is not a concern for workloads. Live migration requires exactly the same CPU on all OpenStack compute nodes, including the CPU microcode and kernel versions. Therefore, for live migrations support, organize your compute nodes into host aggregates and availability zones. For workload migration between non-identical OpenStack compute nodes, contact Mirantis support.

For example, to set the host-passthrough CPU model for all OpenStack compute nodes:

spec:
  features:
    nova:
      vcpu_type: host-passthrough

Custom CPU model

MOSK enables you to specify a comma-separated list of exact QEMU CPU models to create and emulate. Specify the common and less advanced CPU models first. All explicit CPU models provided must be compatible with the OpenStack compute node CPUs.

To specify an exact CPU model, review the available CPU models and their features. List and inspect the /usr/share/libvirt/cpu_map/*.xml files in the libvirt containers of pods of the libvirt DeamonSet or multiple DaemonSets if you are using node-specific settings.

To review the available CPU models

  1. Identify the available libvirt DaemonSets:

    kubectl -n openstack get ds -l application=libvirt --show-labels

    Example of system response:

    NAME                     DESIRED  CURRENT  READY  UP-TO-DATE  AVAILABLE  NODE SELECTOR                   AGE  LABELS
libvirt-libvirt-default  2        2        2      2           2          openstack-compute-node=enabled  34d  app.kubernetes.io/managed-by=Helm,application=libvirt,component=libvirt,release_group=openstack-libvirt

  2. Identify the pods of libvirt DaemonSets:

    kubectl -n openstack get po -l application=libvirt,release_group=openstack-libvirt

    Example of system response:

    NAME                           READY  STATUS   RESTARTS  AGE
libvirt-libvirt-default-5zs8m  2/2    Running  0         8d
libvirt-libvirt-default-vt8wd  2/2    Running  0         3d14h

  3. List and review the available CPU model definition files. For example:

    kubectl -n openstack exec -ti libvirt-libvirt-default-5zs8m -c libvirt -- ls /usr/share/libvirt/cpu_map/*.xml

  4. List and review the content of all CPU model definition files. For example:

    kubectl -n openstack exec -ti libvirt-libvirt-default-5zs8m -c libvirt -- bash -c 'for f in `ls /usr/share/libvirt/cpu_map/*.xml`; do echo $f; cat $f; done'

For example, for nodes that are labeled with processor=amd-epyc, set a custom EPYC CPU model:

spec:
  nodes:
    processor::amd-epyc:
      features:
        nova:
          vcpu_type: EPYC

See also