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


From: Thomas Huth
Subject: Re: [Qemu-devel] [PATCH for-2.7 0/5] spapr: Fix regression in CPU alias handling
Date: Tue, 9 Aug 2016 18:22:05 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2

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




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