Re: [PATCH v5 4/5] crypto: arm64/aes-ccm - remove non-SIMD fallback path

On Mon, 24 May 2021 at 23:51, Eric Biggers [off-list ref] wrote:

On Fri, May 21, 2021 at 12:20:52PM +0200, Ard Biesheuvel wrote:

quoted

AES/CCM on arm64 is implemented as a synchronous AEAD, and so it is
guaranteed by the API that it is only invoked in task or softirq
context. Since softirqs are now only handled when the SIMD is not
being used in the task context that was interrupted to service the
softirq, we no longer need a fallback path. Let's remove it.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/crypto/aes-ce-ccm-core.S |   1 +
 arch/arm64/crypto/aes-ce-ccm-glue.c | 181 ++++++--------------
 2 files changed, 53 insertions(+), 129 deletions(-)

This doesn't just remove the no-SIMD fallback, but it also does some
refactoring.  Notably, it starts to process all the authenticated data in one
kernel_neon_begin() / kernel_neon_end() pair rather than many.  Can you explain
why that is okay now when previously it wasn't, and also split this into two
separate commits?

OK.

For the record, the reason is that, even though kernel_neon_begin/end
are reasonably cheap these days, the common case for CCM (given its
use in networking context) is for the auth/encrypt/finalize routines
to each be called a single time, without any potentially sleeping
calls into the skcipher walk layer in between. Now that we are doing
more work in there (disable softirq processing as well as preemption),
it was a suitable occasion to do some refactoring that I have had on
my list for a while now.