crypto: arm64/sha256-neon - play nice with CONFIG_PREEMPT kernels

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@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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();