Probing for native availability of isel from userspace

[Scott Wood]

On 09/22/2012 08:46:06 PM, Segher Boessenkool wrote:
>>> 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 :-(

The ISA says that isel is "Category: Phased-In (sV2.06)" -- are there  
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  
performance of specific cores rather than some architectural thing,  
maybe that's true with some chips, but on the Freescale side I'd be  
surprised if e5500 were much different from e500v2 in that regard.

-Scott