Re: [PATCH v2 0/6] Faster AES-XTS on modern x86_64 CPUs

Eric Biggers
2024-03-29T09:31:34Z

On Fri, Mar 29, 2024 at 11:03:07AM +0200, Ard Biesheuvel wrote:
>
> Retested this v2:
>
> Tested-by: Ard Biesheuvel <ardb@kernel.org>
> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
>
> Hopefully, the AES-KL keylocker implementation can be based on this
> template as well.

As-is, it would be a bit ugly to add keylocker support to my template because my
template always processes 4 registers of AES blocks per iteration of the main
loop (like the existing aes-xts-aesni), whereas the keylocker instructions are
hardcoded to operate on 8 AES blocks at a time in xmm0-xmm7, presumably to
reduce the overhead of unwrapping the key.

I did try an 8-wide version briefly. There are some older CPUs on which it
helps. (On newer CPUs, AES latency is lower, and the width increases by moving
to ymm or zmm registers anyway.) But it didn't seem too attractive to me. It
causes registers to spill, and it becomes a bit awkward to unroll the AES rounds
when the code size is twice as large, so it may need to be re-rolled. I should
take a closer look, but I decided to just stay with a 4-wide version for now.

So I *think* AES-KL is best kept separate for now. I do wonder if the AES-KL
code should adopt the idea of using VEX-coded instructions, though --- surely
it's the case that in practice, any CPU with AES-KL also supports AVX.

> I wouldn't mind retiring the existing xts(aesni)
> code entirely, and using the xts() wrapper around ecb-aes-aesni on
> 32-bit and on non-AVX uarchs with AES-NI.

Yes, it will need to be benchmarked, but that probably makes sense. If
Wikipedia is to be trusted, on the Intel side only Westmere (from 2010) has
AES-NI but not AVX, and on the AMD side all CPUs with AES-NI have AVX...

- Eric