Re: [PATCH 0/5] crc64: Tweak intrinsics code and enable it for ARM

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Mar 30, 2026 at 04:46:31PM +0200, Ard Biesheuvel wrote:
> Apply some tweaks to the new arm64 crc64 NEON intrinsics code, and wire
> it up for the 32-bit ARM build. Note that true 32-bit ARM CPUs usually
> don't implement the prerequisite 64x64 PMULL instructions, but 32-bit
> kernels are commonly used on 64-bit capable hardware too, which do
> implement the 32-bit versions of the crypto instructions if they are
> implemented for the 64-bit ISA (as per the architecture).
> 
> Cc: Demian Shulhan <demyansh@gmail.com>
> Cc: Eric Biggers <ebiggers@kernel.org>
> 
> Ard Biesheuvel (5):
>   lib/crc: arm64: Drop unnecessary chunking logic from crc64
>   lib/crc: arm64: Use existing macros for kernel-mode FPU cflags
>   ARM: Add a neon-intrinsics.h header like on arm64
>   lib/crc: arm64: Simplify intrinsics implementation
>   lib/crc: arm: Enable arm64's NEON intrinsics implementation of crc64

I think patches 3 and 4 should be swapped, so it's cleanups first (which
make sense regardless of the 32-bit ARM support) and then the 32-bit ARM
support.

I do think we should be aware that even with the code mostly shared
using the NEON intrinsics, the 32-bit ARM support (which works only on
CPUs that support PMULL, i.e. are also 64-bit capable) doesn't come for
free.  We should expect to deal with occasional issues related to the
intrinsics with certain compiler versions, compiler flags, etc.

I assume that "32-bit kernels on ARMv8 CPUs" is currently still a big
enough niche to bother with this, despite that niche getting smaller
over time.  But as I mentioned I do think we should try to simplify it
as much as possible, e.g. by supporting little-endian only and avoiding
#ifdefs based on things like the compiler whenever possible.

- Eric