On Fri, Mar 20, 2015 at 03:25:45PM +0530, Prerna Saxena wrote: > > On Friday 20 March 2015 01:51 PM, Ján Tomko wrote: > > On Mon, Mar 16, 2015 at 04:56:49PM +0530, Prerna Saxena wrote: > >> [PATCH] PowerPC : Do not allow an empty model spec for 'host-model' > >> > >> On PowerPC, a guest VM having CPU mode as 'host-model' > >> represents a 'compat' mode VM. This cannot have a NULL > >> CPU model. > > I thought the compat= mode was only used when mode == HOST_MODEL > > and a model is specified. And HOST_MODEL with no model behaves like on > > x86_64 - copies the features from the host capabilities. > > > > Was this functionality broken by commit addce06 or did it never produce > > useful results? > > > > Jan > > -- > > libvir-list mailing list > > libvir-list@xxxxxxxxxx > > https://www.redhat.com/mailman/listinfo/libvir-list > Hi Jan, > > This commit does not break anything. It addresses a few corner cases not completely addressed by commits addce06 & 5e4f49ab8aa2. > PowerPC pseries KVM is a paravirtualized platform, wherein there are only 2 allowed vcpu configurations of running a guest : > 1) Native mode, where the guest sees the same vcpu model as the host -- this is reflected in libvirt by "host-passthrough" mode; > 2) Compat mode, where the physical processor itself runs in binary compatibility with an older cpu model. This is marked in libvirt by "host-model" mode, which takes on an additional argument -- the > guest CPU model which needs to be run. This was introduced by commit addce06. > I see now that neither libvirt nor QEMU have feature flags for PPC. But there are other machine types than pseries on PPC, some of them might want host-model to generate a -cpu argument, not -cpu compat=. > PowerKVM, being a paravirt platform, does not emulate a guest vcpu based on features copied from host. This behaviour is unlike x86 KVM. Hence , the host-model mode on PowerKVM needs to error out in > case the model which needs to be run in binary compatibility is not specified by user. Instead of erroring out, can it be filled with a sensible value? E.g. on a POWER8 host we fill it out with <model>POWER8</model>. > The reason for this commit was a bug seen even after 5e4f49ab8aa2. A null cpu model XML was causing an incorrect "best-fit" model (just like x86) to be passed to the VM after a save/restore. Hence the > need for this check. So even though the correct model was specified, cpuUpdate removes it after restore? It would be nice to mention that in the commit message. Jan
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