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

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). For determining the actual host-model the QMP command 'query-cpu-model-expansion' is used. This is done once per QEMU binary and the result of it is cached by libvirt. The problem with that is that libvirt queries with the newest machine type of the QEMU binary for the host CPU model.

We could now either probe the host CPU model for each QEMU binary + machine type combination and for this we've to start a new QEMU process each time.