Re: [Qemu-devel] [PATCH] target-i386: fix losing XCR0 processor state component bits

[Eduardo Habkost]

On Wed, Sep 28, 2016 at 01:13:32PM -0300, Eduardo Habkost wrote:
> On Wed, Sep 28, 2016 at 06:07:13PM +0200, Paolo Bonzini wrote:
> > On 28/09/2016 17:59, Eduardo Habkost wrote:
> > > On Wed, Sep 28, 2016 at 05:09:46PM +0200, Paolo Bonzini wrote:
> > >> On 28/09/2016 17:05, Eduardo Habkost wrote:
> > >>>> Hmm, right.  Even though XSAVE could be migrated as a blob, QEMU
> > >>>> marshals and unmarshals the registers out and back into the xsave data,
> > >>>> so that unknown features are indeed unmigratable.
> > >>>>
> > >>>> But are the property names necessary?  It makes no sense to
> > >>>> enable/disable XSAVE components separately from the other CPUID bits
> > >>>> that enable them.  Could we just mark all unknown features as
> > >>>> unmigratable without giving them names?
> > >>>
> > >>> We could, as we don't really need to make them configurable. But
> > >>> giving them names will also allow us to return more useful data
> > >>> to libvirt in case GET_SUPPORTED_CPUID returns some bits as
> > >>> unsupported. The new CPU runnability/comparison APIs are all
> > >>> based on property names.
> > >>
> > >> The names could, or perhaps should, be obtained also from
> > >> x86_ext_save_areas (apart from the legacy x87 and sse components which
> > >> are guaranteed to be there).  Basically property names such as "avx"
> > >> trigger both the regular CPUID bits and the XSAVE components.
> > > 
> > > Yes, this makes sense. If XSTATE_YMM_BIT is missing, for example,
> > > it is more useful to say "avx" is unsupported by the host, than
> > > something like "xsave-component-ymm".
> > 
> > So instead of looking at wi->feat_names[i] we could have a wrapper that
> > returns the name and special cases xsave components words?  This should
> > be used in both x86_cpu_get_migratable_flags and
> > report_unavailable_features, but not elsewhere as far as I could see.
> 
> Sounds good to me. I will do it.

I changed my mind: this will require making a few changes on
ext_save_areas and I want to fix this regression as soon as
possible. I will submit a fix to the regression that adds a
migratable_flags field to FeatureWordInfo, and then implement
your suggestion as part of v2 of my
query-cpu-definitions/unavailable-features series.

-- 
Eduardo