Re: [libvirt] Question about the host-model CPU mode

On Fri, Oct 20, 2017 at 13:37:42 +0200, David Hildenbrand wrote: > On 20.10.2017 13:09, Marc Hartmayer wrote: >> we recently encountered the problem that the 'host-model' [1] has to be >> related to the machine type of a domain. We have following problem: >> >> Let's assume we've a z13 system with a QEMU 2.9 and we define a >> domain using the default s390-virtio-ccw machine together with the >> host-model CPU mode [1]. The definition will have the machine >> expanded to s390-virtio-ccw-2.9 but retain the host-model CPU mode >> in the domain definition. In a next step we upgrade to QEMU 2.10 >> (first version to recognize z14). Everything is still fine, even >> though the machine runs in 2.9 compatibility mode. Finally we >> upgrade to a z14. As a consequence it is not possible to start the >> domain anymore as the machine type doesn't support our CPU host >> model (which is expanded at start time of the domain). > > Yes, the host model may vary depending on QEMU version and to some > degree also on compatibility machines (although I always try to push > people to not introduce any new such stuff). > > Quoting for the libvirt documentation: https://libvirt.org/formatdomain.html > > "host-model > The host-model mode is essentially a shortcut to copying host CPU > definition from capabilities XML into domain XML. Since the CPU > definition is copied just before starting a domain, exactly the same XML > can be used on different hosts while still providing the best guest CPU > each host supports." > > You're telling me that that copying does not happen, which seems to be > the problematic part about this in my opinion. > > So I am not sure if probing anything else (as you noted below) is the > correct thing to do. Copy it and you have a migration_safe and even > static version of the _current_ host CPU. The thing is libvirt calls query-cpu-model-expansion to check what the host CPU is. This 'host-model' CPU is replaced with the probed CPU model when a domain starts. The problem described by Marc is that the probed CPU model cannot be used universally with all machine types. So starting a domain on such host with machine type s390-virtio-ccw-2.10 works, but a domain with machine type s390-virtio-ccw-2.9 fails to start with the same probed CPU model.

Jirka