Re: [Qemu-devel] X86CPU "feature-words" property on QEMU (was Re: [PATCH v3 1/3] x86: Data structure changes to support MSR based) features

[Eduardo Habkost]

On Fri, Aug 17, 2018 at 07:59:40PM +0200, Paolo Bonzini wrote:
> On 17/08/2018 19:48, Eduardo Habkost wrote:
> > So let's do what's necessary to remove it.  But I don't think the
> > removal of "feature-words" should block the inclusion of this
> > series.
> > 
> > Now, should QOM properties follow our feature deprecation policy,
> > or they were never a supported external API and we can remove it
> > immediately?
> > 
> > CCing Jiri and libvir-list, because I just found that there's
> > code on libvirt that uses it, but I don't know exactly it does
> > with that info.
> 
> It is used to check whether the host supports invtsc and pv-unhalt and
> avoid changing the guest ABI when migrating: see
> https://marc.info/?l=libvir-list&m=152445761414746&w=2 for a patch that
> introduces one user.
> 
> I think we should extend it to MSRs, not remove it.

Well, I would prefer if libvirt simply did (e.g.)
"qom-get property=invtsc" instead of fetching raw feature-words
data.  But if we do have an existing user, I now agree with you:
let's keep it working and extend it to include MSR info too.

BTW, libvirt must stop using this hardcoded QOM path:

  #define QOM_CPU_PATH  "/machine/unattached/device[0]"

It should use the "qom_path" field of "query-cpus" instead.

-- 
Eduardo