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

[Eduardo Habkost <ehabkost@redhat.com>]

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".

-- 
Eduardo