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From: Ard Biesheuvel <ardb@kernel.org>
Date: Tue, 11 Jun 2024 17:39:33 +0200
Message-ID: <CAMj1kXEi1LGNu6xwxkAGg_b7DQh4092Y9vb4dobhkB5tbxjsQg@mail.gmail.com>
Subject: Re: [PATCH v5 00/15] Optimize dm-verity and fsverity using
 multibuffer hashing
To: Eric Biggers <ebiggers@kernel.org>
Cc: linux-crypto@vger.kernel.org, fsverity@lists.linux.dev, 
	dm-devel@lists.linux.dev, x86@kernel.org, 
	linux-arm-kernel@lists.infradead.org, Sami Tolvanen <samitolvanen@google.com>, 
	Bart Van Assche <bvanassche@acm.org>, Herbert Xu <herbert@gondor.apana.org.au>
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On Tue, 11 Jun 2024 at 05:49, Eric Biggers <ebiggers@kernel.org> wrote:
>
> On many modern CPUs, it is possible to compute the SHA-256 hash of two
> equal-length messages in about the same time as a single message, if all
> the instructions are interleaved.  This is because each SHA-256 (and
> also most other cryptographic hash functions) is inherently serialized
> and therefore can't always take advantage of the CPU's full throughput.
>
> An earlier attempt to support multibuffer hashing in Linux was based
> around the ahash API.  That approach had some major issues, as does the
> alternative ahash-based approach proposed by Herbert (see my response at
> https://lore.kernel.org/linux-crypto/20240610164258.GA3269@sol.localdomain/).
> This patchset instead takes a much simpler approach of just adding a
> synchronous API for hashing equal-length messages.
>

I share Eric's skepticism that shoehorning this into ahash for
theoretical reasons is going to lead anywhere. So I would strongly
prefer this approach. We can always revisit this if/when this generic
multibuffer ahash materializes.

So for this series

Acked-by: Ard Biesheuvel <ardb@kernel.org>