[PATCH v7 4/7] crypto: arm64/aes-ccm - yield NEON when processing auth-only data

[Ard Biesheuvel <ardb@kernel.org>]

In SIMD accelerated crypto drivers, we typically yield the SIMD unit
after processing 4 KiB of input, to avoid scheduling blackouts caused by
the fact that claiming the SIMD unit disables preemption as well as
softirq processing.

The arm64 CCM driver does this implicitly for the ciphertext, due to the
fact that the skcipher API never processes more than a single page at a
time. However, the scatterwalk performed by this driver when processing
the authenticate-only data will keep the SIMD unit occupied until it
completes.

So cap the scatterwalk steps to 4 KiB.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/crypto/aes-ce-ccm-glue.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/crypto/aes-ce-ccm-glue.c b/arch/arm64/crypto/aes-ce-ccm-glue.c
index f6d19b0dc893..fe9c837ac4b9 100644
--- a/arch/arm64/crypto/aes-ce-ccm-glue.c
+++ b/arch/arm64/crypto/aes-ce-ccm-glue.c
@@ -161,6 +161,7 @@ static void ccm_calculate_auth_mac(struct aead_request *req, u8 mac[])
 			scatterwalk_start(&walk, sg_next(walk.sg));
 			n = scatterwalk_clamp(&walk, len);
 		}
+		n = min_t(u32, n, SZ_4K); /* yield NEON at least every 4k */
 		p = scatterwalk_map(&walk);
 		ccm_update_mac(ctx, mac, p, n, &macp);
 		len -= n;
-- 
2.30.2