On Fri, Apr 19, 2024 at 06:35:01PM +0800, Herbert Xu wrote: > Eric Biggers <ebiggers@xxxxxxxxxx> 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
