Re: [PATCH v3] lib/raid/xor: x86: Add AVX-512 optimized xor_gen()

[Christoph Hellwig <hch@lst.de>]

Can use the xor: prefix used for all other commits to lib/raid/xor?

> Benchmark on AMD Ryzen 9 9950X (Zen 5):
> 
>     src_cnt    avx          avx512       Improvement
>     =======    ==========   ==========   ===========
>     1          56353 MB/s   75388 MB/s   33%
>     2          54274 MB/s   68409 MB/s   26%
>     3          44649 MB/s   64042 MB/s   43%
>     4          41315 MB/s   55002 MB/s   33%

On my Zen 5 mobile (AMD Ryzen AI 7 PRO 350) both the existing
AVX2 and this AVX512 code give numbers in the 200+ GB/s range.  Not
sure if is just the different benchmarking or something else going on.

FYI, one or 2 sources are basically useless as they RAID5 configs
that have no benefits over simple mirroring and thus the numbers
aren't too interesting.

> +DO_XOR_BLOCKS(avx512_inner, xor_avx512_2, xor_avx512_3, xor_avx512_4,
> +	      xor_avx512_5);

Is there really much of a benefit of doing the historic DO_XOR_BLOCKS
vs doing the loop manually?  Especially as the common cases for a
modern RAID will usually loop over more disks than this was built
for.  I.e., in practice one or two source buffers only happen at the
end of a loop over more disks.