io_uring: optimise io_import_iovec fixed path

Delay loading req->rw.{addr,len} in io_import_iovec until it's really
needed, so removing extra loads for the fixed path, which doesn't use
them.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3cc48dd0c4f1a37c4ce9aab5784281a2d83ad8be.1634314022.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3159,9 +3159,9 @@ static int __io_import_iovec(int rw, struct io_kiocb *req, struct iovec **iovec,
 			     struct io_rw_state *s, unsigned int issue_flags)
 {
 	struct iov_iter *iter = &s->iter;
-	void __user *buf = u64_to_user_ptr(req->rw.addr);
-	size_t sqe_len = req->rw.len;
 	u8 opcode = req->opcode;
+	void __user *buf;
+	size_t sqe_len;
 	ssize_t ret;
 
 	if (opcode == IORING_OP_READ_FIXED || opcode == IORING_OP_WRITE_FIXED) {
@@ -3170,9 +3170,12 @@ static int __io_import_iovec(int rw, struct io_kiocb *req, struct iovec **iovec,
 	}
 
 	/* buffer index only valid with fixed read/write, or buffer select  */
-	if (req->buf_index && !(req->flags & REQ_F_BUFFER_SELECT))
+	if (unlikely(req->buf_index && !(req->flags & REQ_F_BUFFER_SELECT)))
 		return -EINVAL;
 
+	buf = u64_to_user_ptr(req->rw.addr);
+	sqe_len = req->rw.len;
+
 	if (opcode == IORING_OP_READ || opcode == IORING_OP_WRITE) {
 		if (req->flags & REQ_F_BUFFER_SELECT) {
 			buf = io_rw_buffer_select(req, &sqe_len, issue_flags);