--- /dev/null
+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, ¶ms, sizeof(params));
+- setsockopt(fd, SOL_NETLINK, NETLINK_TX_RING, ¶ms, 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;
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