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 -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
