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

Eric Biggers
2024-10-25T21:32:44Z

On Fri, Oct 25, 2024 at 10:32:14PM +0200, Ard Biesheuvel wrote:
> On Fri, 25 Oct 2024 at 21:15, Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > From: Eric Biggers <ebiggers@google.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@google.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?

The architecture crc32 modules need to be able to write to this. There could be
a setter function and a getter function, but just using a variable is simpler.

- Eric