Re: [Qemu-devel] [PATCH for-2.7 0/5] spapr: Fix regression in CPU alias handling

[Bharata B Rao]

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

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.

Regards,
Bharata.