Re: [Qemu-devel] [PATCH RESEND v1 1/2] i386: Add Intel Processor Trace feature support

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Jan 15, 2018 at 03:25:18PM +0100, Jiri Denemark wrote:
> On Mon, Jan 15, 2018 at 12:04:55 -0200, Eduardo Habkost wrote:
> > CCing libvirt developers.
> ...
> > This case is slightly more problematic, however: the new feature
> > is actually migratable (under very controlled circumstances)
> > because of patch 2/2, but it is not migration-safe[1].  This
> > means libvirt shouldn't include it in "host-model" expansion
> > (which uses the query-cpu-model-expansion QMP command) until we
> > make the feature migration-safe.
> > 
> > For QEMU, this means the feature shouldn't be returned by
> > "query-cpu-model-expansion type=static model=max" (but it can be
> > returned by "query-cpu-model-expansion type=full model=max").
> > 
> > Jiri, it looks like libvirt uses type=full on
> > query-cpu-model-expansion on x86.  It needs to use
> > type=static[2], or it will have no way to find out if a feature
> > is migration-safe or not.
> ...
> > [2] It looks like libvirt uses type=full because it wants to get
> >     all QOM property aliases returned.  In this case, one
> >     solution for libvirt is to use:
> > 
> >     static_expansion = query_cpu_model_expansion(type=static, model)
> >     all_props = query_cpu_model_expansion(type=full, static_expansion)
> 
> This is exactly what libvirt is doing (with model = "host") ever since
> query-cpu-model-expansion support was implemented for x86.

Oh, now I see that the x86 code uses
QEMU_MONITOR_CPU_MODEL_EXPANSION_STATIC_FULL and not just
QEMU_MONITOR_CPU_MODEL_EXPANSION_FULL.  Nice!

-- 
Eduardo