On 09.08.2016 17:39, Bharata B Rao wrote: > On Tue, Aug 09, 2016 at 11:17:04AM +0200, Thomas Huth wrote: >> There is a regression with the "-cpu" parameter which has been >> introduced by the spapr CPU hotplug code: We used to allow to specify >> a "CPU family" name with the "-cpu" parameter when running on KVM so >> that the user does not need to know the gory details of the exact >> CPU version of the host CPU. For example, it was possible to >> use "-cpu POWER8" on a POWER8E host CPU. This behavior does not >> work anymore with the new hot-pluggable spapr-cpu-core types. >> Since libvirt already heavily depends on the old behavior, this >> is quite a severe regression in the QEMU parameter interface, thus >> I think these patches should still go into 2.7 if possible, to avoid >> that we break the "upper layers" with the final 2.7 release. > > I believed that "-cpu POWER8" on POWER8E host was broken in a way as > the guest CPUs were getting reported as POWER8E instead of POWER8 > > (/proc/cpuinfo of guest) > cpu : POWER8E (raw), altivec supported > > I thought, the correct configuration should have been > > cpu : POWER8 (architected), altivec supported > > which is what you get when you use POWER8 in compat mode on > POWER8E host like below: > > -cpu host -global driver=host-powerpc64-cpu,property=compat,value=power8
As far as I've understood the (old) QEMU source code and the discussions in the past, the "-cpu POWER8" was rather meant as some kind of alias for any CPU in the POWER8 family, i.e. also for POWER8E CPUs. It's just a little bit ugly that "-cpu ?" always lists this as an alias for POWER8_v2.0 - it would be better if we'd somehow update it when we set the POWER8 alias on KVM. > However as you note libvirt is dependent on supporting POWER8 and > there have been discussions and conclusions on this earlier, I guess > it is better now to have your patchset to restore the expectations of > libvirt. Yes, I think we should keep the old behavior now to avoid to break things ... we maybe might want to reconsider the behavior for future CPUs (POWER9?) though. Thomas
