[libvirt] Question about the host-model CPU mode

Marc Hartmayer mhartmay at linux.vnet.ibm.com
Mon Oct 9 08:16:48 UTC 2017

On Thu, Oct 05, 2017 at 02:11 PM +0200, Jiri Denemark <jdenemar at redhat.com> wrote:
> On Thu, Oct 05, 2017 at 10:46:24 +0200, Marc Hartmayer wrote:
>> 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. The definition will have the machine expanded to
>> s390-virtio-ccw-2.9 but retain the host-model CPU mode. 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).
> If I understand it correctly, when QEMU 2.10 is installed, we expand
> the host-model CPU into a different model than with QEMU 2.9. And the
> CPU model we got with QEMU 2.10 does not work with s390-virtio-ccw-2.9
> machine type even on QEMU 2.10. Is that correct?


>> Is this behavior expected?
> Partially.
>> There are some ways to avoid this problem:
>> - CPU model expansion is performed at domain definition time
> No, host-model is designed to be expanded when a domain starts so that
> the same definition always results in the best CPU model to be used no
> matter on which host the domain gest started.
>> - host-model is related to the machine type of a domain
> So if usability of a specific CPU model depends on the selected machine
> types, we probably need to count with this restriction.


> But it's going to be a bit complicated because we ask QEMU what the
> host CPU is and the interface we used would need to be enhanced to
> give us different results for all supported machine types so that we
> can select the right one when a domain is started.

So how do we deal with this?

> Jirka
Beste Grüße / Kind regards
   Marc Hartmayer

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