Probing for native availability of isel from userspace

[Segher Boessenkool]

>> Have a look at /sys/kernel/debug/powerpc/emulated_instructions/ then?
>
> Userspace should *NEVER* rely on the content of debugfs, it will  
> change
> with time, it is not a guaranteed ABI, it's purely for people to look
> at... for debugging.

malc didn't say what he wants it for...  People are in userspace as
well ;-)

> At this stage I would recommend using arch 2.06 as your key/trigger  
> and
> either add a handful of known PVR values (mfpvr is emulated) for other
> CPUs you know support it (there shouldn't be that many), or just do  
> the
> heuristic :-(

That's for 64-bit; another good option for 64-bit is to just never use
isel, it hardly ever buys you anything.  It is much more useful on the
(older) 32-bit cores that support it.

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?


Segher