On Tue, May 08, 2018 at 10:44:22 -0400, Collin Walling wrote:
Hi
I have noticed something that may be misconstrued regarding the libvirt domain xml format for defining a cpu model. There seems to be a misalignment where the libvirt documentation states something that is not supported, but libvirt itself gives no clear indication of such. This is regarding the cpu mode "host-model" and providing a cpu model name between the <model> tags.
From the libvirt docs under header "CPU model and topology" paragraph "cpu" subparagraph "host-model", the following rule is defined (bolded or between asterisks):
"... The match attribute can't be used in this mode. *Specifying CPU model is not supported* either, but model's fallback attribute may still be used. ..."
https://libvirt.org/formatdomain.html#elementsCPU
The above rule reads as "if mode is 'host-model' (and the architecture is not PowerPC) then specifying a model name should not be allowed". However, this is not the observed behavior. For example, I can define and start a guest with the following xml snippet without any issues:
<cpu mode='host-model'> <model>cpu-name</model> </cpu>
Which seems to contradict what the documentation states.
It's not forbidden for compatibility reasons. Old libvirt used to fill in the <model>...</model> in <cpu mode='host-model'></cpu> during migration and save/restore so that the destination would know the actual CPU the domain was started with. We changed this so that host-model automatically turns into mode='custom' CPU when a domain starts, but we still need to support parsing the XML whare mode='host-model' and <model></model> are used at the same time. When a domain is migrated, libvirt will turn the incoming host-model into custom mode. Otherwise the specified model will just be ignored. Jirka