cleanup the RAID5 XOR library

[Christoph Hellwig]

On Fri, Feb 27, 2026 at 11:35:53PM -0800, Eric Biggers wrote:
> >     https://git.infradead.org/?p=users/hch/misc.git;a=shortlog;h=refs/heads/xor-improvements
> 
> Overall this looks great.  xor_gen() really needs a KUnit test, though.
> Without that, how was this tested?

fio data integrity testing on degraded raid.  But yes, a unit test
would be nice.

> Later we should remove some of the obsolete implementations, such as the
> alpha or x86 MMX ones.  Those platforms have no optimized code in
> lib/crc/ or lib/crypto/, and I doubt anyone cares.

I'd rather leave that to the architecture maintainers, but overall I
ahree.

> Also, I notice that no one has optimized this for the latest x86_64 CPUs
> by using the vpternlogd instruction to do 3-input XORs.  That would be
> another good future project.

Yes, as would rewriting the routines to deal with more than 4 + 1 stripes
as that is a really narrow raid these days.