20.41. Configuring the Guest Virtual Machine CPU Model

For simple defaults, the guest virtual machine CPU configuration accepts the same basic XML representation as the host physical machine capabilities XML exposes. In other words, the XML from the virsh cpu-baseline command can now be copied directly into the guest virtual machine XML at the top level under the domain element. In the previous XML snippet, there are a few extra attributes available when describing a CPU in the guest virtual machine XML. These can mostly be ignored, but for the curious here is a quick description of what they do. The top level <cpu> element has an attribute called match with possible values of:

The next enhancement is that the <feature> elements can each have an extra 'policy' attribute with possible values of:

The 'forbid' policy is for a niche scenario where an incorrectly functioning application will try to use a feature even if it is not in the CPUID mask, and you wish to prevent accidentally running the guest virtual machine on a host physical machine with that feature. The 'optional' policy has special behavior with respect to migration. When the guest virtual machine is initially started the flag is optional, but when the guest virtual machine is live migrated, this policy turns into 'require', since you cannot have features disappearing across migration.