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
  processed 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 construction to
  use memory from the TX ring instead of (usually) a buffer declared on the
  stack and setting up the frame header appropriately. 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 continuous memory blocks, containing frames of
  fixed size dependent 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 continuous 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 continuous 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 physically continuous memory for a ring block,
  it will fall back to use physically discontinuous 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
  --------------
  
  As of Jan 2015 the message is always copied from the ring frame to an
  allocated buffer due to unresolved security concerns.
  See commit 4682a0358639b29cf ("netlink: Always copy on mmap TX.").
  
  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, SOL_NETLINK, NETLINK_RX_RING, &req, sizeof(req)) < 0)
  		exit(1);
  	if (setsockopt(fd, SOL_NETLINK, NETLINK_TX_RING, &req, sizeof(req)) < 0)
  		exit(1)
  
  	/* Calculate size of each individual 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;