Re: [PATCH] lib/crc: arm64: use .arch_extension to enable CRC and crypto

["Ard Biesheuvel" <ardb@kernel.org>]

Hello Emanuele,

On Thu, 5 Nov 2026, at 16:41, Emanuele Rocca wrote:
> Instead of using ".cpu generic+crc+crypto", use ".arch_extension crc" and
> ".arch_extension crypto" to enable the required instructions.
>
> The ".cpu generic+crc+crypto" directive replaces the assembler's current
> feature set with generic+crc+crypto, dropping any other features that were
> previously enabled, for example by -march or by a toolchain configured with
> --with-arch=armv9-a. In contrast, ".arch_extension" enables the requested
> extensions while preserving the current feature set.
>
> This is not a functional change: crc32-core.S only uses base A64
> instructions plus CRC32 and PMULL, which remain available, and BTI landing
> pads are emitted as HINT instructions. However, using .arch_extension
> avoids the pitfall of silently disabling features. See commit 9142c2f8f84b
> ("octeontx2: use arch_extension to enable LSE").
>

I don't mind the patch, but I will point out that issues such as these can
never occur in .S sources. The emitted instructions are always the same, and
the only reason we need the .arch is so that the assembler permits the use
of the special ones.

Commit 9142c2f8f84b is different because it uses .arch in inline asm, where
it switches the assembler into a different mode behind the back of the
compiler, leading it to reject instructions generated by the compiler.