On 7/10/25 6:10 PM, Eric Biggers wrote:
On Thu, Jul 10, 2025 at 10:28:55AM +0200, Milan Broz wrote:
On 7/9/25 9:09 PM, Eric Biggers wrote:
The support for asynchronous hashes in dm-verity has outlived its
usefulness.  It adds significant code complexity and opportunity for
bugs.  I don't know of anyone using it in practice.  (The original
submitter of the code possibly was, but that was 8 years ago.)  Data I
recently collected for en/decryption shows that using off-CPU crypto
"accelerators" is consistently much slower than the CPU
(https://lore.kernel.org/r/20250704070322.20692-1-ebigg...@kernel.org/),
even on CPUs that lack dedicated cryptographic instructions.  Similar
results are likely to be seen for hashing.

I already removed support for asynchronous hashes from fsverity two
years ago, and no one ever complained.

Moreover, neither dm-verity, fsverity, nor fscrypt has ever actually
used the asynchronous crypto algorithms in a truly asynchronous manner.
The lack of interest in such optimizations provides further evidence
that it's only the CPU-based crypto that actually matters.

Historically, it's also been common for people to forget to enable the
optimized SHA-256 code, which could contribute to an off-CPU crypto
engine being perceived as more useful than it really is.  In 6.16 I
fixed that: the optimized SHA-256 code is now enabled by default.

Therefore, let's drop the support for asynchronous hashes in dm-verity.

Tested with verity-compat-test.

Hi,

I shortly tested it with veritysetup too, also on 32bit.
And I like this patch (I wish we can remove the async thing from the dmcrypt 
too...)

IMO we should do it for dm-crypt too, though it's going to be a slightly
tougher sell there because it actually goes through the trouble of using
the async API "properly".

We tested async by replacing dm-crypt/dm-verity arg with cryptd(%s-generic).
Nice, if it really works, but... I think it is time to simplify it.

Thanks,
Milan



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