On Tue, May 08, 2018 at 10:44:22AM -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>
After starting the guest, you still see the same as above when you do `virsh dumpxml $GUEST`? Also does the "cpu-name" of your choice really shows up when check the QEMU command-line for the guest?
Which seems to contradict what the documentation states.
This issue was reported by a colleague of mine who was confused by the cpu features that were available to a guest when host-model and a model name are provided. Personally, I tend to err on the side of providing host-model and a cpu-model-name being mutually exclusive.
I've attempted to find a solution to this problem myself by looking at virCPUDefParseXML, but the fact that PowerPC exists as an exception and we do not know the architecture when parsing a guest cpu xml makes minimal code changes challenging.
If we want to make changes to the code, then I imagine that the ideal solution would revolve around only allowing <model>cpu-name</model> to be valid iff the cpu mode is set to "custom". Otherwise some clarity on the documentation would suffice. Something like "A CPU model specified in the domain xml will be ignored." Thoughts?
I'd wait for Jiri Denemark to chime in with a more detailed response. While at it, that whole 'host-model' section could be clarified and updated to be more readable (if you wrap that at 72 lines, it ends up being a huge 30-line unreadable paragraph). I keep intending to do that, but never got around so far. -- /kashyap