Re: [RFC PATCH 1/8] crypto: shash - add support for finup2x

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Apr 19, 2024 at 06:35:01PM +0800, Herbert Xu wrote:
> Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > The new API is part of the "shash" algorithm type, as it does not make
> > sense in "ahash".  It does a "finup" operation rather than a "digest"
> > operation in order to support the salt that is used by dm-verity and
> > fs-verity.  There is no fallback implementation that does two regular
> > finups if the underlying algorithm doesn't support finup2x, since users
> > probably will want to avoid the overhead of queueing up multiple hashes
> > when multibuffer hashing won't actually be used anyway.
> 
> For your intended users, will the SIMD fallback ever be invoked?
> 

If you mean the fallback to scalar instructions when !crypto_simd_usable(), by
default dm-verity and fs-verity do all hashing in process context, in which case
the scalar fallback will never be used.  dm-verity does support the
'try_verify_in_tasklet' option which makes hashing sometimes happen in softirq
context, and x86 Linux has an edge case where if a softirq comes in while the
kernel is in the middle of using SIMD instructions, SIMD instructions can't be
used during that softirq.  So in theory the !crypto_simd_usable() case could be
reached then.  Either way, I have the fallback implemented in the x86 and arm64
SHA-256 glue code for consistency with the rest of the crypto_shash API anyway.

If you mean falling back to two crypto_shash_finup() when the algorithm doesn't
support crypto_shash_finup2x(), my patches to dm-verity and fs-verity do that.
Modern x86_64 and arm64 systems will use crypto_shash_finup2x(), but dm-verity
and fs-verity need to work on all architectures and on older CPUs too.  The
alternative would be to put the fallback to two crypto_shash_finup() directly in
crypto_shash_finup2x() and have the users call crypto_shash_finup2x()
unconditionally (similar to how crypto_shash_digest() can be called even if the
underlying shash_alg doesn't implement ->digest()).  That would make for
slightly simpler code, though it feels a bit awkward to queue up multiple blocks
for multibuffer hashing when multibuffer hashing won't actually be used.  Let me
know if you have a preference about this.

- Eric