Context
I'm running a virtualisation lab environment, with four Intel-based CPUs (i5-6500) and one AMD Ryzen 3 PRO 2200GE.
When migrating virtual machines from one of the Intel hosts to the AMD host, the migration would fail with the following error:
error: operation failed: guest CPU doesn't match specification: extra features: cmp_legacy"
The really strange thing is that some virtual machines did migrate successfully from an Intel system to the AMD system without any issues.
So I dumped the XML of the virtual machines and compared them. It turns out that the virtual machines that migrated successfully were all single-core and all failing virtual machines were multi-core.
As the cmp_legacy CPU feature seems to be related to (lack of) support for hyper-threading, the configured CPU topology of the virtual machines was suspected to be the culprit.
I've configured a custom CPU topology on the virtual machines with 1 socket, 1 die and one or more CPU cores. This caused the migration failure.
Solution
If you configure multi-core virtual machines with multiple sockets instead of cores migration between Intel and AMD hosts will succeed.
In the past, migrations between an Intel and AMD host always succeeded, assuming that the appropriate CPU features were disabled. Why this suddenly stopped working is not clear to me, as this configuration never changed. Maybe it's related to a Libvirt update, but I have no proof for this.
