Re: [PATCH v4 08/21] lib/crc: Switch ARM and arm64 to 'ksimd' scoped guard API

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Fri, 31 Oct 2025 11:39:07 +0100
Ard Biesheuvel <ardb+git@google.com> wrote:

> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Before modifying the prototypes of kernel_neon_begin() and
> kernel_neon_end() to accommodate kernel mode FP/SIMD state buffers
> allocated on the stack, move arm64 to the new 'ksimd' scoped guard API,
> which encapsulates the calls to those functions.
> 
> For symmetry, do the same for 32-bit ARM too.
> 
> Reviewed-by: Eric Biggers <ebiggers@kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  lib/crc/arm/crc-t10dif.h   | 16 +++++-----------
>  lib/crc/arm/crc32.h        | 11 ++++-------
>  lib/crc/arm64/crc-t10dif.h | 16 +++++-----------
>  lib/crc/arm64/crc32.h      | 16 ++++++----------
>  4 files changed, 20 insertions(+), 39 deletions(-)
> 
> diff --git a/lib/crc/arm/crc-t10dif.h b/lib/crc/arm/crc-t10dif.h
> index 63441de5e3f1..aaeeab0defb5 100644
> --- a/lib/crc/arm/crc-t10dif.h
> +++ b/lib/crc/arm/crc-t10dif.h

>  static __ro_after_init DEFINE_STATIC_KEY_FALSE(have_neon);
> @@ -20,21 +19,16 @@ asmlinkage void crc_t10dif_pmull8(u16 init_crc, const u8 *buf, size_t len,
>  static inline u16 crc_t10dif_arch(u16 crc, const u8 *data, size_t length)
>  {
>  	if (length >= CRC_T10DIF_PMULL_CHUNK_SIZE) {
> -		if (static_branch_likely(&have_pmull)) {
> -			if (likely(may_use_simd())) {
> -				kernel_neon_begin();
> -				crc = crc_t10dif_pmull64(crc, data, length);
> -				kernel_neon_end();
> -				return crc;
> -			}
> +		if (static_branch_likely(&have_pmull) && likely(may_use_simd())) {
> +			scoped_ksimd()
> +				return crc_t10dif_pmull64(crc, data, length);

>  		} else if (length > CRC_T10DIF_PMULL_CHUNK_SIZE &&
>  			   static_branch_likely(&have_neon) &&
>  			   likely(may_use_simd())) {

I briefly thought this was a functional change but it's not because
of may_use_simd() being something that isn't going to change between
the two evaluations.  

Would it hurt at all to pull that up to be
	if (length >= CRC_T10DIF_PMULL_CHUNK_SIZE && likely(may_use_simd())) {
		if (static_branch_likely(&have_pmull)) {
			scoped_ksimd()
				return crc_t10dif_pmull64(crc, data, length);

 		} else if (length > CRC_T10DIF_PMULL_CHUNK_SIZE &&
 			   static_branch_likely(&have_neon)) {
		...

?