Re: [PATCH v2 00/12] lib/crc: improve how arch-optimized code is integrated

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Jun 09, 2025 at 06:15:24PM +1000, Julian Calaby wrote:
> Hi Eric,
> 
> On Sun, Jun 8, 2025 at 6:07 AM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > This series is also available at:
> >
> >     git fetch https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git lib-crc-arch-v2
> >
> > This series improves how lib/crc supports arch-optimized code.  First,
> > instead of the arch-optimized CRC code being in arch/$(SRCARCH)/lib/, it
> > will now be in lib/crc/$(SRCARCH)/.  Second, the API functions (e.g.
> > crc32c()), arch-optimized functions (e.g. crc32c_arch()), and generic
> > functions (e.g. crc32c_base()) will now be part of a single module for
> > each CRC type, allowing better inlining and dead code elimination.  The
> > second change is made possible by the first.
> >
> > As an example, consider CONFIG_CRC32=m on x86.  We'll now have just
> > crc32.ko instead of both crc32-x86.ko and crc32.ko.  The two modules
> > were already coupled together and always both got loaded together via
> > direct symbol dependency, so the separation provided no benefit.
> >
> > Note: later I'd like to apply the same design to lib/crypto/ too, where
> > often the API functions are out-of-line so this will work even better.
> > In those cases, for each algorithm we currently have 3 modules all
> > coupled together, e.g. libsha256.ko, libsha256-generic.ko, and
> > sha256-x86.ko.  We should have just one, inline things properly, and
> > rely on the compiler's dead code elimination to decide the inclusion of
> > the generic code instead of manually setting it via kconfig.
> >
> > Having arch-specific code outside arch/ was somewhat controversial when
> > Zinc proposed it back in 2018.  But I don't think the concerns are
> > warranted.  It's better from a technical perspective, as it enables the
> > improvements mentioned above.  This model is already successfully used
> > in other places in the kernel such as lib/raid6/.  The community of each
> > architecture still remains free to work on the code, even if it's not in
> > arch/.  At the time there was also a desire to put the library code in
> > the same files as the old-school crypto API, but that was a mistake; now
> > that the library is separate, that's no longer a constraint either.
> 
> Quick question, and apologies if this has been covered elsewhere.
> 
> Why not just use choice blocks in Kconfig to choose the compiled-in
> crc32 variant instead of this somewhat indirect scheme?
>
> This would keep the dependencies grouped by arch and provide a single place to
> choose whether the generic or arch-specific method is used.

It's not clear exactly what you're suggesting, but it sounds like you're
complaining about this:

    config CRC32_ARCH
            bool
            depends on CRC32 && CRC_OPTIMIZATIONS
            default y if ARM && KERNEL_MODE_NEON
            default y if ARM64
            default y if LOONGARCH
            default y if MIPS && CPU_MIPSR6
            default y if PPC64 && ALTIVEC
            default y if RISCV && RISCV_ISA_ZBC
            default y if S390
            default y if SPARC64
            default y if X86

We could instead make each arch be responsible for selecting this from
lib/crc/$(SRCARCH)/Kconfig, which lib/crc/Kconfig would then have to include.
But I don't think the small bit of additional per-arch separation would be worth
the extra complexity here.  Something similar applies to lib/crc/Makefile too.

This patchset strikes a balance where the vast majority of the arch-specific CRC
code is isolated in lib/crc/$(SRCARCH), and the exceptions are just
lib/crc/Makefile and lib/crc/Kconfig.  I think these exceptions make sense,
given that we're building a single module per CRC variant.  We'd have to go
through some hoops to isolate the arch-specific Kconfig and Makefile snippets
into per-arch files, which don't seem worth it here IMO.

> It would also allow for alternatives if that ever becomes a thing and

If you mean one arch with multiple alternative implementations of a particular
CRC variant, that already exists for many of the architectures.  They just build
in as many as can be, and the best one is chosen at boot or module load time.

But that's existing behavior, unchanged by this patchset.

> compile testing of the arch-specific variants if that even offers any
> actual value.

They all use instructions specific to the corresponding arch, so I don't think
any of them would be compatible with COMPILE_TEST.

- Eric