[PATCH v2 10/19] crypto: arm64/sha256-neon - play nice with CONFIG_PREEMPT kernels

[Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>]

Tweak the SHA256 update routines to invoke the SHA256 block transform
block by block, to avoid excessive scheduling delays caused by the
NEON algorithm running with preemption disabled.

Also, remove a stale comment which no longer applies now that kernel
mode NEON is actually disallowed in some contexts.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
 arch/arm64/crypto/sha256-glue.c | 36 +++++++++++++-------
 1 file changed, 23 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/crypto/sha256-glue.c b/arch/arm64/crypto/sha256-glue.c
index b064d925fe2a..e8880ccdc71f 100644
--- a/arch/arm64/crypto/sha256-glue.c
+++ b/arch/arm64/crypto/sha256-glue.c
@@ -89,21 +89,32 @@ static struct shash_alg algs[] = { {
 static int sha256_update_neon(struct shash_desc *desc, const u8 *data,
 			      unsigned int len)
 {
-	/*
-	 * Stacking and unstacking a substantial slice of the NEON register
-	 * file may significantly affect performance for small updates when
-	 * executing in interrupt context, so fall back to the scalar code
-	 * in that case.
-	 */
+	struct sha256_state *sctx = shash_desc_ctx(desc);
+
 	if (!may_use_simd())
 		return sha256_base_do_update(desc, data, len,
 				(sha256_block_fn *)sha256_block_data_order);
 
-	kernel_neon_begin();
-	sha256_base_do_update(desc, data, len,
-				(sha256_block_fn *)sha256_block_neon);
-	kernel_neon_end();
+	while (len > 0) {
+		unsigned int chunk = len;
+
+		/*
+		 * Don't hog the CPU for the entire time it takes to process all
+		 * input when running on a preemptible kernel, but process the
+		 * data block by block instead.
+		 */
+		if (IS_ENABLED(CONFIG_PREEMPT) &&
+		    chunk + sctx->count % SHA256_BLOCK_SIZE > SHA256_BLOCK_SIZE)
+			chunk = SHA256_BLOCK_SIZE -
+				sctx->count % SHA256_BLOCK_SIZE;
 
+		kernel_neon_begin();
+		sha256_base_do_update(desc, data, chunk,
+				      (sha256_block_fn *)sha256_block_neon);
+		kernel_neon_end();
+		data += chunk;
+		len -= chunk;
+	}
 	return 0;
 }
 
@@ -117,10 +128,9 @@ static int sha256_finup_neon(struct shash_desc *desc, const u8 *data,
 		sha256_base_do_finalize(desc,
 				(sha256_block_fn *)sha256_block_data_order);
 	} else {
-		kernel_neon_begin();
 		if (len)
-			sha256_base_do_update(desc, data, len,
-				(sha256_block_fn *)sha256_block_neon);
+			sha256_update_neon(desc, data, len);
+		kernel_neon_begin();
 		sha256_base_do_finalize(desc,
 				(sha256_block_fn *)sha256_block_neon);
 		kernel_neon_end();
-- 
2.11.0