performance: memcpy vs. __copy_tofrom_user

[Scott Wood]

Benjamin Herrenschmidt wrote:
>> It doesn't need to be done in non-preemptible sections, if you have a
>> separate per-thread save area for kernel fp/altivec use (and appropriate
>> flags so an FP unavailable handler knows which regs to restore), and you
>> can avoid using it in a preempting context.
> 
> Yuck.

Hmm?  It's simple and achieves the desired result (avoiding 
non-preemptible regions without unduly restricting the ability to 
extract performance from the hardware).

Would it be nicer to avoid FP/Altivec in the kernel altogether?  Sure. 
If the benchmarking says that we're better off with it, though, then so 
be it.

-Scott