Re: [PATCH 0/4] RISC-V CRC optimizations

Ignacio Encinas Rubio
2025-03-08T12:58:52Z

Hello!

On 2/3/25 23:04, Eric Biggers wrote:
> So, quite positive results. Though, the fact the msb-first CRCs are (still) so
> much slower than lsb-first ones indicates that be64_to_cpu() is super slow on
> RISC-V. That seems to be caused by the rev8 instruction from Zbb not being
> used. I wonder if there are any plans to make the endianness swap macros use
> rev8, or if I'm going to have to roll my own endianness swap in the CRC code.
> (I assume it would be fine for the CRC code to depend on both Zbb and Zbc.)

I saw this message the other day and started working on a patch, but I
would like to double-check I'm on the right track:

- be64_to_cpu ends up being __swab64 (include/uapi/linux/swab.h)

If Zbb was part of the base ISA, turning CONFIG_ARCH_USE_BUILTIN_BSWAP
would take care of the problem, but it is not the case.

Therefore, we have to define __arch_swab<X> like some "arches"(?) do in
arch/<ARCH>/include/uapi/asm/swab.h

For those functions to be correct in generic kernels, we would need to
use ALTERNATIVE() macros like in arch/riscv/include/asm/bitops.h.
Would this be ok? I'm not sure if the ALTERNATIVEs overhead can be a
problem here.

Thanks in advance :)