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 09:40:40AM +0200, Ingo Molnar wrote:
> 
> * 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.
> > 
> > Changed in v2:
> >    - Fixed build warning on architectures without any optimized CRC code
> >    - Fixed build warning in sparc/crc32.h by removing pr_fmt
> >    - Moved fallback definitions of crc32*_arch back into arch files
> >    - Remove ARCH_HAS_CRC* symbols at end of series instead of beginning,
> >      so that they're not removed until they're no longer being selected
> >    - Slightly improved some commit messages
> >    - Rebased onto other pending lib/crc changes
> > 
> > Eric Biggers (12):
> >   lib/crc: move files into lib/crc/
> >   lib/crc: prepare for arch-optimized code in subdirs of lib/crc/
> >   lib/crc/arm: migrate arm-optimized CRC code into lib/crc/
> >   lib/crc/arm64: migrate arm64-optimized CRC code into lib/crc/
> >   lib/crc/loongarch: migrate loongarch-optimized CRC code into lib/crc/
> >   lib/crc/mips: migrate mips-optimized CRC code into lib/crc/
> >   lib/crc/powerpc: migrate powerpc-optimized CRC code into lib/crc/
> >   lib/crc/riscv: migrate riscv-optimized CRC code into lib/crc/
> >   lib/crc/s390: migrate s390-optimized CRC code into lib/crc/
> >   lib/crc/sparc: migrate sparc-optimized CRC code into lib/crc/
> >   lib/crc/x86: migrate x86-optimized CRC code into lib/crc/
> >   lib/crc: remove ARCH_HAS_* kconfig symbols
> 
> For the movement of the x86 bits:
> 
>   Acked-by: Ingo Molnar <mingo@kernel.org>
> 
> >  rename {arch/s390/lib => lib/crc/s390}/crc32be-vx.c (100%)
> >  rename {arch/s390/lib => lib/crc/s390}/crc32le-vx.c (100%)
> >  rename arch/sparc/lib/crc32.c => lib/crc/sparc/crc32.h (60%)
> >  rename {arch/sparc/lib => lib/crc/sparc}/crc32c_asm.S (100%)
> >  create mode 100644 lib/crc/tests/Makefile
> >  rename lib/{ => crc}/tests/crc_kunit.c (100%)
> >  rename {arch/x86/lib => lib/crc/x86}/crc-pclmul-consts.h (100%)
> >  rename {arch/x86/lib => lib/crc/x86}/crc-pclmul-template.S (100%)
> >  rename {arch/x86/lib => lib/crc/x86}/crc-pclmul-template.h (100%)
> >  rename arch/x86/lib/crc-t10dif.c => lib/crc/x86/crc-t10dif.h (56%)
> >  rename {arch/x86/lib => lib/crc/x86}/crc16-msb-pclmul.S (100%)
> >  rename {arch/x86/lib => lib/crc/x86}/crc32-pclmul.S (100%)
> 
> One small namespace suggestion: wouldn't it be better to move the arch 
> support code to lib/crc/arch/, instead of lib/crc/? That way any 
> generic code will stand out better and architecture directories don't 
> crowd out what is supposed to be generic code.

I don't think that yet another level of directories would provide much value
here.  The only non-arch subdirectory of lib/crc/ is "tests", so it's not like
there are a lot of subdirectories that could be confused with arch names.

- Eric