[libvirt] Question about the host-model CPU mode

[Jiri Denemark]

On Fri, Oct 20, 2017 at 13:09:26 +0200, 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).
> 
> 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.

No, libvirt probes QEMU with -machine none.

> 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.

This is not really a viable solution.

Jirka