Re: [PATCH] crypto: x86/aes-xts - access round keys using single-byte offsets

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Apr 09, 2024 at 11:12:11AM +0200, Ard Biesheuvel wrote:
> On Tue, 9 Apr 2024 at 02:02, Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > From: Eric Biggers <ebiggers@google.com>
> >
> > Access the AES round keys using offsets -7*16 through 7*16, instead of
> > 0*16 through 14*16.  This allows VEX-encoded instructions to address all
> > round keys using 1-byte offsets, whereas before some needed 4-byte
> > offsets.  This decreases the code size of aes-xts-avx-x86_64.o by 4.2%.
> >
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> 
> Nice optimization!
> 
> Do you think we might be able to macrofy this a bit so we can use zero
> based indexing for the round keys, and hide the arithmetic?
> 
> 

There are two alternatives I considered: defining variables KEYOFF0 through
KEYOFF14 and writing the offsets as KEYOFF\i(KEY), or defining one variable
KEYOFF and writing the offsets as \i*16-KEYOFF(KEY).  I think I slightly prefer
the current patch where it's less abstracted out, though.  It makes it clear the
offsets really are single-byte, and also index 7 is the exact mid-point so going
from -7 to 7 still feels fairly natural.  If we wanted to do something more
complex like use different offsets for AVX vs. AVX512, then we'd need the
abstraction to handle that, but it doesn't seem useful to do that.

- Eric