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

From: Eric Biggers <[email-masked]>
Date: Fri, 25 Oct 2024 21:32:43 +0000

On Fri, Oct 25, 2024 at 10:32:14PM +0200, Ard Biesheuvel wrote:
> On Fri, 25 Oct 2024 at 21:15, Eric Biggers <[email-masked]> wrote:
> >
> > From: Eric Biggers <[email-masked]>
> >
> > 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 <[email-masked]>
> > ---
> >  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