crypto: x86/aes-xts - make the fast path 64-bit specific

Remove 32-bit support from the fast path in xts_crypt().  Then optimize
it for 64-bit, and simplify the code, by switching to sg_virt() and
removing the now-unnecessary checks for crossing a page boundary.

The result is simpler code that is slightly smaller and faster in the
case that actually matters (64-bit).

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Author: ebiggers

arch/x86/crypto/aesni-intel_glue.c

@@ -581,35 +581,25 @@ xts_crypt(struct skcipher_request *req, xts_encrypt_iv_func encrypt_iv,
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
 	const struct aesni_xts_ctx *ctx = aes_xts_ctx(tfm);
-	const unsigned int cryptlen = req->cryptlen;
-	struct scatterlist *src = req->src;
-	struct scatterlist *dst = req->dst;
 
-	if (unlikely(cryptlen < AES_BLOCK_SIZE))
+	if (unlikely(req->cryptlen < AES_BLOCK_SIZE))
 		return -EINVAL;
 
 	kernel_fpu_begin();
 	(*encrypt_iv)(&ctx->tweak_ctx, req->iv);
 
 	/*
 	 * In practice, virtually all XTS plaintexts and ciphertexts are either
-	 * 512 or 4096 bytes, aligned such that they don't span page boundaries.
-	 * To optimize the performance of these cases, and also any other case
-	 * where no page boundary is spanned, the below fast-path handles
-	 * single-page sources and destinations as efficiently as possible.
+	 * 512 or 4096 bytes and do not use multiple scatterlist elements.  To
+	 * optimize the performance of these cases, the below fast-path handles
+	 * single-scatterlist-element messages as efficiently as possible.  The
+	 * code is 64-bit specific, as it assumes no page mapping is needed.
 	 */
-	if (likely(src->length >= cryptlen && dst->length >= cryptlen &&
-		   src->offset + cryptlen <= PAGE_SIZE &&
-		   dst->offset + cryptlen <= PAGE_SIZE)) {
-		struct page *src_page = sg_page(src);
-		struct page *dst_page = sg_page(dst);
-		void *src_virt = kmap_local_page(src_page) + src->offset;
-		void *dst_virt = kmap_local_page(dst_page) + dst->offset;
-
-		(*crypt_func)(&ctx->crypt_ctx, src_virt, dst_virt, cryptlen,
-			      req->iv);
-		kunmap_local(dst_virt);
-		kunmap_local(src_virt);
+	if (IS_ENABLED(CONFIG_X86_64) &&
+	    likely(req->src->length >= req->cryptlen &&
+		   req->dst->length >= req->cryptlen)) {
+		(*crypt_func)(&ctx->crypt_ctx, sg_virt(req->src),
+			      sg_virt(req->dst), req->cryptlen, req->iv);
 		kernel_fpu_end();
 		return 0;
 	}