Re: [PATCH v4 6/8] fsverity: improve performance by using multibuffer hashing

Herbert Xu
2024-06-11T15:21:49Z

On Mon, Jun 10, 2024 at 09:42:58AM -0700, Eric Biggers wrote:
>
> I understand that you think the ahash based API would make it easier to add
> multibuffer support to "authenc(hmac(sha256),cbc(aes))" for IPsec, which seems
> to be a very important use case for you (though it isn't relevant to nearly as
> many systems as dm-verity and fsverity are). Regardless, the reality is that it
> would be much more difficult to take advantage of multibuffer crypto in the
> IPsec authenc use case than in dm-verity and fsverity. authenc uses multiple
> underlying algorithms, AES-CBC and HMAC-SHA256, that would both have to use
> multibuffer crypto in order to see a significant benefit, seeing as even if the
> SHA-256 support could be wired up through HMAC-SHA256, encryption would be
> bottlenecked on AES-CBC, especially on Intel CPUs. It also looks like the IPsec
> code would need a lot of updates to support multibuffer crypto.

The linked-request thing feeds nicely into networking. In fact
that's where I got the idea of linking them from. In networking
a large GSO (currently limited to 64K but theoretically we could
make it unlimited) packet is automatically split up into a linked
list of MTU-sized skb's.

Therefore if we switched to a linked-list API networking could
give us the buffers with minimal changes.

BTW, I found an old Intel paper that claims through their multi-
buffer strategy they were able to make AES-CBC-XCBC beat AES-GCM.
I wonder if we could still replicate this today:

https://github.com/intel/intel-ipsec-mb/wiki/doc/fast-multi-buffer-ipsec-implementations-ia-processors-paper.pdf

> Ultimately, I need to have dm-verity and fsverity be properly optimized in the
> downstreams that are most relevant to me. If you're not going to allow the
> upstream crypto API to provide the needed functionality in a reasonable way,
> then I'll need to shift my focus to getting this patchset into downstream
> kernels such as Android and Chrome OS instead.

I totally understand that this is your priority. But please give
me some time to see if we can devise something that works for both
scenarios.

Thanks,