Subject: Re: Probing for native availability of isel from userspace

From: Scott Wood <[email-masked]>
Date: Mon, 24 Sep 2012 18:55:09 -0500

On 09/22/2012 08:46:06 PM, Segher Boessenkool wrote:
>>> Have a look at /sys/kernel/debug/powerpc/emulated_instructions/ =20
>>> then?
>>=20
>> Userspace should *NEVER* rely on the content of debugfs, it will =20
>> change
>> with time, it is not a guaranteed ABI, it's purely for people to look
>> at... for debugging.
>=20
> malc didn't say what he wants it for...  People are in userspace as
> well ;-)
>=20
>> At this stage I would recommend using arch 2.06 as your key/trigger =20
>> and
>> either add a handful of known PVR values (mfpvr is emulated) for =20
>> other
>> CPUs you know support it (there shouldn't be that many), or just do =20
>> the
>> heuristic :-(

The ISA says that isel is "Category: Phased-In (sV2.06)" -- are there =20
any 2.06 chips that don't have it?

> 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 is it more useful on 32-bit?  If you're referring to the =20
performance of specific cores rather than some architectural thing, =20
maybe that's true with some chips, but on the Freescale side I'd be =20
surprised if e5500 were much different from e500v2 in that regard.

-Scott=