Goal

Libvirt support "host-model" CPU model which is widely used to create a migratable CPU model which closely matches host CPU. Hyper-V enlightenments, however, are not included and thus must be listed separately. Currently, there is <hyperv mode> setting which can either be custom or passthrough. The suggestion is to add third mode: host which would expand to the list of supported Hyper-V enlightenments.

Background information

Mostly, Hyper-V enlightenments are vendor-agnostic, however, there are some vendor specific ones. In particular, Enlightened VMCS can only be enabled on Intel hosts. Then, there are additional enlightenments (e.g. Direct TLB flush) which require eVMCS. Upper layers of virt stack now have to carry two separate templates: for AMD and Intel to get all the Hyper-V features or stick to the common subset, this is inconvenient.

Note: libvirt can query supported Hyper-V capabilities since:

commit 10f47848648941c21ca8816458b33a37eb3a6d34
Author: Michal Prívozník <mprivozn@redhat.com>
Date:   Mon Dec 19 15:48:02 2022 +0100
    qemu_capabilities: Query for Hyper-V Enlightenments

Acceptance criteria

• It is possible to create a migratable CPU model which picks all the currently present enlightenments on the host.