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

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Jun 17, 2026 at 07:56:53AM +0200, Christoph Hellwig wrote:
> 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.

I used lib/raid/xor/xor-core.c which measures the throughput of parity
data generated, whereas your proposed xor_benchmark() in xor_kunit
measures the throughput of source data consumed.  I don't know which
makes more sense, but we should make them consistent with each other.

> 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.

There's not really a way out of unrolling by source buffer count, as
otherwise the pointers would continuously have to be reloaded into
registers.  That's why your proposal was so slow (see the numbers I gave
in https://lore.kernel.org/linux-crypto/20260612055933.GA6675@sol/ ).
It could be something different from 2-5 specifically, or open-coded
instead of using the macro if that's all you're asking for, but at a
high level the unrolling by source buffer count does seem to be needed.

- Eric