Re: [PATCH 0/7] crypto: switch to static calls for CRC-T10DIF

[Ard Biesheuvel <ardb@kernel.org>]

On Mon, 11 Jan 2021 at 17:52, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> CRC-T10DIF is a very poor match for the crypto API:
> - every user in the kernel calls it via a library wrapper around the
>   shash API, so all callers share a single instance of the transform
> - each architecture provides at most a single optimized implementation,
>   based on SIMD instructions for carryless multiplication
>
> In other words, none of the flexibility it provides is put to good use,
> and so it is better to get rid of this complexity, and expose the optimized
> implementations via static calls instead. This removes a substantial chunk
> of code, and also gets rid of any indirect calls on architectures that
> obsess about those (x86)
>
> If this approach is deemed suitable, there are other places where we might
> consider adopting it: CRC32 and CRC32(C).
>
> Patch #1 does some preparatory refactoring and removes the library wrapper
> around the shash transform.
>
> Patch #2 introduces the actual static calls, along with the registration
> routines to update the crc-t10dif implementation at runtime.
>
> Patch #3 updates the generic CRC-T10DIF shash driver so it distinguishes
> between the optimized library code and the generic library code.
>
> Patches #4 to #7 update the various arch implementations to switch over to
> the new method.
>
> Special request to Peter to take a look at patch #2, and in particular,
> whether synchronize_rcu_tasks() is sufficient to ensure that a module
> providing the target of a static call can be unloaded safely.
>

It seems I may have managed to confuse myself slightly here: without
an upper bound on the size of the input of the crc_t10dif() routine, I
suppose we can never assume that all its callers have finished.

Insights welcome ...