netlink: add documentation for memory mapped I/O

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/netlink_mmap.txt

+This file documents how to use memory mapped I/O with netlink.
+
+Author: Patrick McHardy <kaber@trash.net>
+
+Overview
+--------
+
+Memory mapped netlink I/O can be used to increase throughput and decrease
+overhead of unicast receive and transmit operations. Some netlink subsystems
+require high throughput, these are mainly the netfilter subsystems
+nfnetlink_queue and nfnetlink_log, but it can also help speed up large
+dump operations of f.i. the routing database.
+
+Memory mapped netlink I/O used two circular ring buffers for RX and TX which
+are mapped into the processes address space.
+
+The RX ring is used by the kernel to directly construct netlink messages into
+user-space memory without copying them as done with regular socket I/O,
+additionally as long as the ring contains messages no recvmsg() or poll()
+syscalls have to be issued by user-space to get more message.
+
+The TX ring is used to process messages directly from user-space memory, the
+kernel processes all messages contained in the ring using a single sendmsg()
+call.
+
+Usage overview
+--------------
+
+In order to use memory mapped netlink I/O, user-space needs three main changes:
+
+- ring setup
+- conversion of the RX path to get messages from the ring instead of recvmsg()
+- conversion of the TX path to construct messages into the ring
+
+Ring setup is done using setsockopt() to provide the ring parameters to the
+kernel, then a call to mmap() to map the ring into the processes address space:
+
+- setsockopt(fd, SOL_NETLINK, NETLINK_RX_RING, &params, sizeof(params));
+- setsockopt(fd, SOL_NETLINK, NETLINK_TX_RING, &params, sizeof(params));
+- ring = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)
+
+Usage of either ring is optional, but even if only the RX ring is used the
+mapping still needs to be writable in order to update the frame status after
+processing.
+
+Conversion of the reception path involves calling poll() on the file
+descriptor, once the socket is readable the frames from the ring are
+processsed in order until no more messages are available, as indicated by
+a status word in the frame header.
+
+On kernel side, in order to make use of memory mapped I/O on receive, the
+originating netlink subsystem needs to support memory mapped I/O, otherwise
+it will use an allocated socket buffer as usual and the contents will be
+ copied to the ring on transmission, nullifying most of the performance gains.
+Dumps of kernel databases automatically support memory mapped I/O.
+
+Conversion of the transmit path involves changing message contruction to
+use memory from the TX ring instead of (usually) a buffer declared on the
+stack and setting up the frame header approriately. Optionally poll() can
+be used to wait for free frames in the TX ring.
+
+Structured and definitions for using memory mapped I/O are contained in
+<linux/netlink.h>.
+
+RX and TX rings
+----------------
+
+Each ring contains a number of continous memory blocks, containing frames of
+fixed size dependant on the parameters used for ring setup.
+
+Ring:	[ block 0 ]
+		[ frame 0 ]
+		[ frame 1 ]
+	[ block 1 ]
+		[ frame 2 ]
+		[ frame 3 ]
+	...
+	[ block n ]
+		[ frame 2 * n ]
+		[ frame 2 * n + 1 ]
+
+The blocks are only visible to the kernel, from the point of view of user-space
+the ring just contains the frames in a continous memory zone.
+
+The ring parameters used for setting up the ring are defined as follows:
+
+struct nl_mmap_req {
+	unsigned int	nm_block_size;
+	unsigned int	nm_block_nr;
+	unsigned int	nm_frame_size;
+	unsigned int	nm_frame_nr;
+};
+
+Frames are grouped into blocks, where each block is a continous region of memory
+and holds nm_block_size / nm_frame_size frames. The total number of frames in
+the ring is nm_frame_nr. The following invariants hold:
+
+- frames_per_block = nm_block_size / nm_frame_size
+
+- nm_frame_nr = frames_per_block * nm_block_nr
+
+Some parameters are constrained, specifically:
+
+- nm_block_size must be a multiple of the architectures memory page size.
+  The getpagesize() function can be used to get the page size.
+
+- nm_frame_size must be equal or larger to NL_MMAP_HDRLEN, IOW a frame must be
+  able to hold at least the frame header
+
+- nm_frame_size must be smaller or equal to nm_block_size
+
+- nm_frame_size must be a multiple of NL_MMAP_MSG_ALIGNMENT
+
+- nm_frame_nr must equal the actual number of frames as specified above.
+
+When the kernel can't allocate phsyically continous memory for a ring block,
+it will fall back to use physically discontinous memory. This might affect
+performance negatively, in order to avoid this the nm_frame_size parameter
+should be chosen to be as small as possible for the required frame size and
+the number of blocks should be increased instead.
+
+Ring frames
+------------
+
+Each frames contain a frame header, consisting of a synchronization word and some
+meta-data, and the message itself.
+
+Frame:	[ header message ]
+
+The frame header is defined as follows:
+
+struct nl_mmap_hdr {
+	unsigned int	nm_status;
+	unsigned int	nm_len;
+	__u32		nm_group;
+	/* credentials */
+	__u32		nm_pid;
+	__u32		nm_uid;
+	__u32		nm_gid;
+};
+
+- nm_status is used for synchronizing processing between the kernel and user-
+  space and specifies ownership of the frame as well as the operation to perform
+
+- nm_len contains the length of the message contained in the data area
+
+- nm_group specified the destination multicast group of message
+
+- nm_pid, nm_uid and nm_gid contain the netlink pid, UID and GID of the sending
+  process. These values correspond to the data available using SOCK_PASSCRED in
+  the SCM_CREDENTIALS cmsg.
+
+The possible values in the status word are:
+
+- NL_MMAP_STATUS_UNUSED:
+	RX ring:	frame belongs to the kernel and contains no message
+			for user-space. Approriate action is to invoke poll()
+			to wait for new messages.
+
+	TX ring:	frame belongs to user-space and can be used for
+			message construction.
+
+- NL_MMAP_STATUS_RESERVED:
+	RX ring only:	frame is currently used by the kernel for message
+			construction and contains no valid message yet.
+			Appropriate action is to invoke poll() to wait for
+			new messages.
+
+- NL_MMAP_STATUS_VALID:
+	RX ring:	frame contains a valid message. Approriate action is
+			to process the message and release the frame back to
+			the kernel by setting the status to
+			NL_MMAP_STATUS_UNUSED or queue the frame by setting the
+			status to NL_MMAP_STATUS_SKIP.
+
+	TX ring:	the frame contains a valid message from user-space to
+			be processed by the kernel. After completing processing
+			the kernel will release the frame back to user-space by
+			setting the status to NL_MMAP_STATUS_UNUSED.
+
+- NL_MMAP_STATUS_COPY:
+	RX ring only:	a message is ready to be processed but could not be
+			stored in the ring, either because it exceeded the
+			frame size or because the originating subsystem does
+			not support memory mapped I/O. Appropriate action is
+			to invoke recvmsg() to receive the message and release
+			the frame back to the kernel by setting the status to
+			NL_MMAP_STATUS_UNUSED.
+
+- NL_MMAP_STATUS_SKIP:
+	RX ring only:	user-space queued the message for later processing, but
+			processed some messages following it in the ring. The
+			kernel should skip this frame when looking for unused
+			frames.
+
+The data area of a frame begins at a offset of NL_MMAP_HDRLEN relative to the
+frame header.
+
+TX limitations
+--------------
+
+Kernel processing usually involves validation of the message received by
+user-space, then processing its contents. The kernel must assure that
+userspace is not able to modify the message contents after they have been
+validated. In order to do so, the message is copied from the ring frame
+to an allocated buffer if either of these conditions is false:
+
+- only a single mapping of the ring exists
+- the file descriptor is not shared between processes
+
+This means that for threaded programs, the kernel will fall back to copying.
+
+Example
+-------
+
+Ring setup:
+
+	unsigned int block_size = 16 * getpagesize();
+	struct nl_mmap_req req = {
+		.nm_block_size		= block_size,
+		.nm_block_nr		= 64,
+		.nm_frame_size		= 16384,
+		.nm_frame_nr		= 64 * block_size / 16384,
+	};
+	unsigned int ring_size;
+	void *rx_ring, *tx_ring;
+
+	/* Configure ring parameters */
+	if (setsockopt(fd, NETLINK_RX_RING, &req, sizeof(req)) < 0)
+		exit(1);
+	if (setsockopt(fd, NETLINK_TX_RING, &req, sizeof(req)) < 0)
+		exit(1)
+
+	/* Calculate size of each invididual ring */
+	ring_size = req.nm_block_nr * req.nm_block_size;
+
+	/* Map RX/TX rings. The TX ring is located after the RX ring */
+	rx_ring = mmap(NULL, 2 * ring_size, PROT_READ | PROT_WRITE,
+		       MAP_SHARED, fd, 0);
+	if ((long)rx_ring == -1L)
+		exit(1);
+	tx_ring = rx_ring + ring_size:
+
+Message reception:
+
+This example assumes some ring parameters of the ring setup are available.
+
+	unsigned int frame_offset = 0;
+	struct nl_mmap_hdr *hdr;
+	struct nlmsghdr *nlh;
+	unsigned char buf[16384];
+	ssize_t len;
+
+	while (1) {
+		struct pollfd pfds[1];
+
+		pfds[0].fd	= fd;
+		pfds[0].events	= POLLIN | POLLERR;
+		pfds[0].revents	= 0;
+
+		if (poll(pfds, 1, -1) < 0 && errno != -EINTR)
+			exit(1);
+
+		/* Check for errors. Error handling omitted */
+		if (pfds[0].revents & POLLERR)
+			<handle error>
+
+		/* If no new messages, poll again */
+		if (!(pfds[0].revents & POLLIN))
+			continue;
+
+		/* Process all frames */
+		while (1) {
+			/* Get next frame header */
+			hdr = rx_ring + frame_offset;
+
+			if (hdr->nm_status == NL_MMAP_STATUS_VALID)
+				/* Regular memory mapped frame */
+				nlh = (void *hdr) + NL_MMAP_HDRLEN;
+				len = hdr->nm_len;
+
+				/* Release empty message immediately. May happen
+				 * on error during message construction.
+				 */
+				if (len == 0)
+					goto release;
+			} else if (hdr->nm_status == NL_MMAP_STATUS_COPY) {
+				/* Frame queued to socket receive queue */
+				len = recv(fd, buf, sizeof(buf), MSG_DONTWAIT);
+				if (len <= 0)
+					break;
+				nlh = buf;
+			} else
+				/* No more messages to process, continue polling */
+				break;
+
+			process_msg(nlh);
+release:
+			/* Release frame back to the kernel */
+			hdr->nm_status = NL_MMAP_STATUS_UNUSED;
+
+			/* Advance frame offset to next frame */
+			frame_offset = (frame_offset + frame_size) % ring_size;
+		}
+	}
+
+Message transmission:
+
+This example assumes some ring parameters of the ring setup are available.
+A single message is constructed and transmitted, to send multiple messages
+at once they would be constructed in consecutive frames before a final call
+to sendto().
+
+	unsigned int frame_offset = 0;
+	struct nl_mmap_hdr *hdr;
+	struct nlmsghdr *nlh;
+	struct sockaddr_nl addr = {
+		.nl_family	= AF_NETLINK,
+	};
+
+	hdr = tx_ring + frame_offset;
+	if (hdr->nm_status != NL_MMAP_STATUS_UNUSED)
+		/* No frame available. Use poll() to avoid. */
+		exit(1);
+
+	nlh = (void *)hdr + NL_MMAP_HDRLEN;
+
+	/* Build message */
+	build_message(nlh);
+
+	/* Fill frame header: length and status need to be set */
+	hdr->nm_len	= nlh->nlmsg_len;
+	hdr->nm_status	= NL_MMAP_STATUS_VALID;
+
+	if (sendto(fd, NULL, 0, 0, &addr, sizeof(addr)) < 0)
+		exit(1);
+
+	/* Advance frame offset to next frame */
+	frame_offset = (frame_offset + frame_size) % ring_size;