performance: memcpy vs. __copy_tofrom_user

[Paul Mackerras]

Dominik Bozek writes:

> Actually I made couple of other tests on that mpc8313. Most of them are
> to ugly to publish them, but... My problem is that I have to boost the
> gigabit interface on the mpc8313. I made simple substitution and
> __copy_tofrom_user was used instead of memcpy. I know, it's wrong, but I
> speedup that way the network interface for about 10%.

Very interesting.  Can you work out where memcpy is being called on
the network data?  I wouldn't have expected that.

There is actually no strong reason not to use __copy_tofrom_user as
memcpy, in fact, as long as we are sure that source and destination
are both cacheable.

Paul.