linux-kernel - Re: [PATCH 3/7] crypto: powerpc/md5 - Remove PowerPC optimized MD5 code

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Message-ID: <20250803225652.GA2142@quark>
Date: Sun, 3 Aug 2025 15:56:52 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Segher Boessenkool <segher@...nel.crashing.org>
Cc: linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>,
	"Jason A . Donenfeld" <Jason@...c4.com>, linux-mips@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH 3/7] crypto: powerpc/md5 - Remove PowerPC optimized MD5
 code

On Sun, Aug 03, 2025 at 05:27:01PM -0500, Segher Boessenkool wrote:
> You are replacing a known-working target implementation by a lower
> performance generic implementation.

That's probably correct, though FWIW there have been quite a few cases
where optimized assembly code in the kernel actually turned out to be
slower than the C code.  (That primarily happens when the assembly code
doesn't take advantage of any special CPU features, which was the case
for this PowerPC code.)  I don't have PowerPC hardware to check the
exact performance differential here, but IMO even if there was a
slowdown the factors still weigh strongly in favor of retiring this.

> But is that one known-working at all?  Does it come with tests?  Was
> it tested to have the same outputs as the existing thing, maybe?  Just
> on a few inputs maybe.

Of course.  Patch 7 adds a KUnit test suite for MD5, and there are still
the older tests in crypto/testmgr.c.  And of course generic code is much
easier to test than arch-specific code.  So not only is it tested, but
the test coverage is much better than it was before.

- Eric