Probing for native availability of isel from userspace

[Benjamin Herrenschmidt]

On Mon, 2012-09-24 at 09:55 +0200, Gabriel Paubert wrote:
> On Sun, Sep 23, 2012 at 03:46:06AM +0200, Segher Boessenkool wrote:
> > Why does the kernel emulate this, btw?  I can see emulation is useful
> > for running older binaries, for instructions that have been removed
> > from the architecture; but for newly added instructions, or optional
> > instructions, it hurts more than it helps?
> 
> Indeed. I also don't understand why mfpvr is emulated. That's the kind
> of information that should be passed to the executables through auxiliary
> vectors. After all, you can (or could at least) compile a kernel without
> Altivec support and run it on a processor with Altivec.
> 
> Therefore, whether Altivec is supported or not, is a matter of
> processor and kernel options. Provide this information through
> the auxiliary vector and the problem is solved.

Which we do. mfpvr is available as a fallback (essentially because if we
don't do it somebody's going to parse /proc/cpuinfo which is arguably
worse :-)

We should definitely advertise the availability of isel.

Cheers,
Ben.