performance: memcpy vs. __copy_tofrom_user
Scott Wood
scottwood at freescale.com
Tue Oct 14 08:03:30 EST 2008
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
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