linux-kernel - Re: [PATCH v2 03/18] lib/crc32: expose whether the lib is really optimized at runtime

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Message-ID: <CAMj1kXFoer+_yZJWtqBVYfYnzqL9X9bbBRomCL3LDqRcYJ6njQ@mail.gmail.com>
Date: Fri, 25 Oct 2024 22:32:14 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org, 
	linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net, 
	linux-mips@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	linux-s390@...r.kernel.org, linux-scsi@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev, 
	sparclinux@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2 03/18] lib/crc32: expose whether the lib is really
 optimized at runtime

On Fri, 25 Oct 2024 at 21:15, Eric Biggers <ebiggers@...nel.org> wrote:
>
> From: Eric Biggers <ebiggers@...gle.com>
>
> Make the CRC32 library export some flags that indicate which CRC32
> functions are actually executing optimized code at runtime.  Set these
> correctly from the architectures that implement the CRC32 functions.
>
> This will be used to determine whether the crc32[c]-$arch shash
> algorithms should be registered in the crypto API.  btrfs could also
> start using these flags instead of the hack that it currently uses where
> it parses the crypto_shash_driver_name.
>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  arch/arm64/lib/crc32-glue.c  | 15 +++++++++++++++
>  arch/riscv/lib/crc32-riscv.c | 15 +++++++++++++++
>  include/linux/crc32.h        | 15 +++++++++++++++
>  lib/crc32.c                  |  5 +++++
>  4 files changed, 50 insertions(+)
>
...
> diff --git a/include/linux/crc32.h b/include/linux/crc32.h
> index 58c632533b08..bf26d454b60d 100644
> --- a/include/linux/crc32.h
> +++ b/include/linux/crc32.h
> @@ -35,10 +35,25 @@ static inline u32 __pure __crc32c_le(u32 crc, const u8 *p, size_t len)
>         if (IS_ENABLED(CONFIG_CRC32_ARCH))
>                 return crc32c_le_arch(crc, p, len);
>         return crc32c_le_base(crc, p, len);
>  }
>
> +/*
> + * crc32_optimizations contains flags that indicate which CRC32 library
> + * functions are using architecture-specific optimizations.  Unlike
> + * IS_ENABLED(CONFIG_CRC32_ARCH) it takes into account the different CRC32
> + * variants and also whether any needed CPU features are available at runtime.
> + */
> +#define CRC32_LE_OPTIMIZATION  BIT(0) /* crc32_le() is optimized */
> +#define CRC32_BE_OPTIMIZATION  BIT(1) /* crc32_be() is optimized */
> +#define CRC32C_OPTIMIZATION    BIT(2) /* __crc32c_le() is optimized */
> +#if IS_ENABLED(CONFIG_CRC32_ARCH)
> +extern u32 crc32_optimizations;
> +#else
> +#define crc32_optimizations 0
> +#endif
> +

Wouldn't it be cleaner to add a new library function for this, instead
of using a global variable?

>  /**
>   * crc32_le_combine - Combine two crc32 check values into one. For two
>   *                   sequences of bytes, seq1 and seq2 with lengths len1
>   *                   and len2, crc32_le() check values were calculated
>   *                   for each, crc1 and crc2.
> diff --git a/lib/crc32.c b/lib/crc32.c
> index 47151624332e..194de73f30f8 100644
> --- a/lib/crc32.c
> +++ b/lib/crc32.c
> @@ -336,5 +336,10 @@ u32 __pure crc32_be_base(u32 crc, const u8 *p, size_t len)
>  {
>         return crc32_be_generic(crc, p, len, crc32table_be, CRC32_POLY_BE);
>  }
>  #endif
>  EXPORT_SYMBOL(crc32_be_base);
> +
> +#if IS_ENABLED(CONFIG_CRC32_ARCH)
> +u32 crc32_optimizations;
> +EXPORT_SYMBOL(crc32_optimizations);
> +#endif
> --
> 2.47.0
>
>