Comment 1 for bug 1982853

This is really just a nice to have option for operators that find them selves in an unfortunate situation where they need to move vms to an old hypervisor e.g. rolling back an upgrade or unexpected hardware failure in the middle of an rolling upgrade.

it is not a bug in nova itself nor is it really a feature since it not something we would ever enable by default.

so triaging this as wishlist since its nice to have but not strictly required.
you can use cold migration in this case and if the VM is using a feature form the new hypervisor release that will still be required.

as noted libvirt will do its own internal check and if the VM is using a feature not supported by the dest host it will still reject the live migration. so this is safe but suboptimal since this would make the failure much later in this case.

for operator that are manually intervening and know that the VM will work on the dest the scheduling pessimisation is not a concern and since livemigation is an admin only operation i think this is oke as a workaround.