raid5 xor functions and caches

[Andi Kleen <ak@muc.de>]

Hi,

I tuned the xor.h functions for x86-64 a bit. One of the changes I did
was to use streaming stores, which are usually faster.  But it's actually
slower in the test it does a booting because that tests cache hot behaviour.
NTI will invalidate the cache line, so it's eating a lot of cache misses.

Does it make sense to test cache hot behaviour? iirc RAID checksumming
should be done without polluting caches. I'm considering to change it
to benchmark a few MB of data buffers to avoid caching effects. 

Also I'm not quite sure about the logic between XOR_SELECT_TEMPLATE prefering
SSE :- if the CPU supports SSE2 then all functions including integer can be 
compiled with NTA prefetches and NTI stores (on some CPUs an integer NTI
store is not a good idea though) For distribution kernels
they can be duplicated. If integer is really faster for some things
it should be used this way.

Another thing I noticed is that the prefetch distance for the SSE
functions is a bit too short. On modern CPUs 256 bytes is not long enough
(on P4 that would be only two cachelines), it needs more (3-4 cachelines at
least). 

-Andi