io_uring: add support for IORING_OP_MSG_RING command

This adds support for IORING_OP_MSG_RING, which allows an SQE to signal
another ring. That allows either waking up someone waiting on the ring,
or even passing a 64-bit value via the user_data field in the CQE.

sqe->fd must contain the fd of a ring that should receive the CQE.
sqe->off will be propagated to the cqe->user_data on the target ring,
and sqe->len will be propagated to cqe->res. The results CQE will have
IORING_CQE_F_MSG set in its flags, to indicate that this CQE was generated
from a messaging request rather than a SQE issued locally on that ring.
This effectively allows passing a 64-bit and a 32-bit quantify between
the two rings.

This request type has the following request specific error cases:

- -EBADFD. Set if the sqe->fd doesn't point to a file descriptor that is
  of the io_uring type.
- -EOVERFLOW. Set if we were not able to deliver a request to the target
  ring.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -706,6 +706,12 @@ struct io_hardlink {
 	int				flags;
 };
 
+struct io_msg {
+	struct file			*file;
+	u64 user_data;
+	u32 len;
+};
+
 struct io_async_connect {
 	struct sockaddr_storage		address;
 };
@@ -871,6 +877,7 @@ struct io_kiocb {
 		struct io_mkdir		mkdir;
 		struct io_symlink	symlink;
 		struct io_hardlink	hardlink;
+		struct io_msg		msg;
 	};
 
 	u8				opcode;
@@ -1121,6 +1128,9 @@ static const struct io_op_def io_op_defs[] = {
 	[IORING_OP_MKDIRAT] = {},
 	[IORING_OP_SYMLINKAT] = {},
 	[IORING_OP_LINKAT] = {},
+	[IORING_OP_MSG_RING] = {
+		.needs_file		= 1,
+	},
 };
 
 /* requests with any of those set should undergo io_disarm_next() */
@@ -4322,6 +4332,46 @@ static int io_nop(struct io_kiocb *req, unsigned int issue_flags)
 	return 0;
 }
 
+static int io_msg_ring_prep(struct io_kiocb *req,
+			    const struct io_uring_sqe *sqe)
+{
+	if (unlikely(sqe->addr || sqe->ioprio || sqe->buf_index ||
+		     sqe->rw_flags || sqe->splice_fd_in || sqe->buf_index ||
+		     sqe->personality))
+		return -EINVAL;
+
+	if (req->file->f_op != &io_uring_fops)
+		return -EBADFD;
+
+	req->msg.user_data = READ_ONCE(sqe->off);
+	req->msg.len = READ_ONCE(sqe->len);
+	return 0;
+}
+
+static int io_msg_ring(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_ring_ctx *target_ctx;
+	struct io_msg *msg = &req->msg;
+	int ret = -EOVERFLOW;
+	bool filled;
+
+	target_ctx = req->file->private_data;
+
+	spin_lock(&target_ctx->completion_lock);
+	filled = io_fill_cqe_aux(target_ctx, msg->user_data, msg->len,
+					IORING_CQE_F_MSG);
+	io_commit_cqring(target_ctx);
+	spin_unlock(&target_ctx->completion_lock);
+
+	if (filled) {
+		io_cqring_ev_posted(target_ctx);
+		ret = 0;
+	}
+
+	__io_req_complete(req, issue_flags, ret, 0);
+	return 0;
+}
+
 static int io_fsync_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_ring_ctx *ctx = req->ctx;
@@ -6700,6 +6750,8 @@ static int io_req_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return io_symlinkat_prep(req, sqe);
 	case IORING_OP_LINKAT:
 		return io_linkat_prep(req, sqe);
+	case IORING_OP_MSG_RING:
+		return io_msg_ring_prep(req, sqe);
 	}
 
 	printk_once(KERN_WARNING "io_uring: unhandled opcode %d\n",
@@ -6983,6 +7035,9 @@ static int io_issue_sqe(struct io_kiocb *req, unsigned int issue_flags)
 	case IORING_OP_LINKAT:
 		ret = io_linkat(req, issue_flags);
 		break;
+	case IORING_OP_MSG_RING:
+		ret = io_msg_ring(req, issue_flags);
+		break;
 	default:
 		ret = -EINVAL;
 		break;

include/uapi/linux/io_uring.h

@@ -143,6 +143,7 @@ enum {
 	IORING_OP_MKDIRAT,
 	IORING_OP_SYMLINKAT,
 	IORING_OP_LINKAT,
+	IORING_OP_MSG_RING,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,
@@ -199,9 +200,11 @@ struct io_uring_cqe {
  *
  * IORING_CQE_F_BUFFER	If set, the upper 16 bits are the buffer ID
  * IORING_CQE_F_MORE	If set, parent SQE will generate more CQE entries
+ * IORING_CQE_F_MSG	If set, CQE was generated with IORING_OP_MSG_RING
  */
 #define IORING_CQE_F_BUFFER		(1U << 0)
 #define IORING_CQE_F_MORE		(1U << 1)
+#define IORING_CQE_F_MSG		(1U << 2)
 
 enum {
 	IORING_CQE_BUFFER_SHIFT		= 16,