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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>
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Patrick McHardy
David S. Miller
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339
Documentation/networking/netlink_mmap.txt
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339
Documentation/networking/netlink_mmap.txt
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This file documents how to use memory mapped I/O with netlink.
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Author: Patrick McHardy <kaber@trash.net>
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Overview
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--------
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Memory mapped netlink I/O can be used to increase throughput and decrease
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overhead of unicast receive and transmit operations. Some netlink subsystems
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require high throughput, these are mainly the netfilter subsystems
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nfnetlink_queue and nfnetlink_log, but it can also help speed up large
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dump operations of f.i. the routing database.
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Memory mapped netlink I/O used two circular ring buffers for RX and TX which
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are mapped into the processes address space.
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The RX ring is used by the kernel to directly construct netlink messages into
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user-space memory without copying them as done with regular socket I/O,
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additionally as long as the ring contains messages no recvmsg() or poll()
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syscalls have to be issued by user-space to get more message.
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The TX ring is used to process messages directly from user-space memory, the
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kernel processes all messages contained in the ring using a single sendmsg()
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call.
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Usage overview
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--------------
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In order to use memory mapped netlink I/O, user-space needs three main changes:
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- ring setup
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- conversion of the RX path to get messages from the ring instead of recvmsg()
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- conversion of the TX path to construct messages into the ring
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Ring setup is done using setsockopt() to provide the ring parameters to the
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kernel, then a call to mmap() to map the ring into the processes address space:
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- setsockopt(fd, SOL_NETLINK, NETLINK_RX_RING, ¶ms, sizeof(params));
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- setsockopt(fd, SOL_NETLINK, NETLINK_TX_RING, ¶ms, sizeof(params));
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- ring = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)
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Usage of either ring is optional, but even if only the RX ring is used the
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mapping still needs to be writable in order to update the frame status after
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processing.
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Conversion of the reception path involves calling poll() on the file
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descriptor, once the socket is readable the frames from the ring are
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processsed in order until no more messages are available, as indicated by
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a status word in the frame header.
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On kernel side, in order to make use of memory mapped I/O on receive, the
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originating netlink subsystem needs to support memory mapped I/O, otherwise
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it will use an allocated socket buffer as usual and the contents will be
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copied to the ring on transmission, nullifying most of the performance gains.
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Dumps of kernel databases automatically support memory mapped I/O.
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Conversion of the transmit path involves changing message contruction to
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use memory from the TX ring instead of (usually) a buffer declared on the
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stack and setting up the frame header approriately. Optionally poll() can
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be used to wait for free frames in the TX ring.
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Structured and definitions for using memory mapped I/O are contained in
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<linux/netlink.h>.
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RX and TX rings
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----------------
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Each ring contains a number of continous memory blocks, containing frames of
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fixed size dependant on the parameters used for ring setup.
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Ring: [ block 0 ]
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[ frame 0 ]
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[ frame 1 ]
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[ block 1 ]
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[ frame 2 ]
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[ frame 3 ]
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...
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[ block n ]
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[ frame 2 * n ]
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[ frame 2 * n + 1 ]
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The blocks are only visible to the kernel, from the point of view of user-space
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the ring just contains the frames in a continous memory zone.
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The ring parameters used for setting up the ring are defined as follows:
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struct nl_mmap_req {
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unsigned int nm_block_size;
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unsigned int nm_block_nr;
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unsigned int nm_frame_size;
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unsigned int nm_frame_nr;
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};
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Frames are grouped into blocks, where each block is a continous region of memory
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and holds nm_block_size / nm_frame_size frames. The total number of frames in
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the ring is nm_frame_nr. The following invariants hold:
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- frames_per_block = nm_block_size / nm_frame_size
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- nm_frame_nr = frames_per_block * nm_block_nr
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Some parameters are constrained, specifically:
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- nm_block_size must be a multiple of the architectures memory page size.
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The getpagesize() function can be used to get the page size.
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- nm_frame_size must be equal or larger to NL_MMAP_HDRLEN, IOW a frame must be
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able to hold at least the frame header
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- nm_frame_size must be smaller or equal to nm_block_size
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- nm_frame_size must be a multiple of NL_MMAP_MSG_ALIGNMENT
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- nm_frame_nr must equal the actual number of frames as specified above.
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When the kernel can't allocate phsyically continous memory for a ring block,
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it will fall back to use physically discontinous memory. This might affect
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performance negatively, in order to avoid this the nm_frame_size parameter
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should be chosen to be as small as possible for the required frame size and
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the number of blocks should be increased instead.
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Ring frames
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------------
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Each frames contain a frame header, consisting of a synchronization word and some
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meta-data, and the message itself.
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Frame: [ header message ]
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The frame header is defined as follows:
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struct nl_mmap_hdr {
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unsigned int nm_status;
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unsigned int nm_len;
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__u32 nm_group;
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/* credentials */
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__u32 nm_pid;
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__u32 nm_uid;
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__u32 nm_gid;
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};
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- nm_status is used for synchronizing processing between the kernel and user-
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space and specifies ownership of the frame as well as the operation to perform
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- nm_len contains the length of the message contained in the data area
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- nm_group specified the destination multicast group of message
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- nm_pid, nm_uid and nm_gid contain the netlink pid, UID and GID of the sending
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process. These values correspond to the data available using SOCK_PASSCRED in
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the SCM_CREDENTIALS cmsg.
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The possible values in the status word are:
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- NL_MMAP_STATUS_UNUSED:
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RX ring: frame belongs to the kernel and contains no message
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for user-space. Approriate action is to invoke poll()
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to wait for new messages.
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TX ring: frame belongs to user-space and can be used for
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message construction.
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- NL_MMAP_STATUS_RESERVED:
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RX ring only: frame is currently used by the kernel for message
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construction and contains no valid message yet.
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Appropriate action is to invoke poll() to wait for
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new messages.
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- NL_MMAP_STATUS_VALID:
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RX ring: frame contains a valid message. Approriate action is
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to process the message and release the frame back to
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the kernel by setting the status to
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NL_MMAP_STATUS_UNUSED or queue the frame by setting the
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status to NL_MMAP_STATUS_SKIP.
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TX ring: the frame contains a valid message from user-space to
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be processed by the kernel. After completing processing
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the kernel will release the frame back to user-space by
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setting the status to NL_MMAP_STATUS_UNUSED.
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- NL_MMAP_STATUS_COPY:
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RX ring only: a message is ready to be processed but could not be
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stored in the ring, either because it exceeded the
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frame size or because the originating subsystem does
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not support memory mapped I/O. Appropriate action is
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to invoke recvmsg() to receive the message and release
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the frame back to the kernel by setting the status to
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NL_MMAP_STATUS_UNUSED.
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- NL_MMAP_STATUS_SKIP:
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RX ring only: user-space queued the message for later processing, but
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processed some messages following it in the ring. The
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kernel should skip this frame when looking for unused
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frames.
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The data area of a frame begins at a offset of NL_MMAP_HDRLEN relative to the
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frame header.
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TX limitations
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--------------
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Kernel processing usually involves validation of the message received by
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user-space, then processing its contents. The kernel must assure that
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userspace is not able to modify the message contents after they have been
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validated. In order to do so, the message is copied from the ring frame
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to an allocated buffer if either of these conditions is false:
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- only a single mapping of the ring exists
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- the file descriptor is not shared between processes
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This means that for threaded programs, the kernel will fall back to copying.
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Example
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-------
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Ring setup:
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unsigned int block_size = 16 * getpagesize();
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struct nl_mmap_req req = {
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.nm_block_size = block_size,
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.nm_block_nr = 64,
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.nm_frame_size = 16384,
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.nm_frame_nr = 64 * block_size / 16384,
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};
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unsigned int ring_size;
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void *rx_ring, *tx_ring;
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/* Configure ring parameters */
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if (setsockopt(fd, NETLINK_RX_RING, &req, sizeof(req)) < 0)
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exit(1);
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if (setsockopt(fd, NETLINK_TX_RING, &req, sizeof(req)) < 0)
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exit(1)
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/* Calculate size of each invididual ring */
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ring_size = req.nm_block_nr * req.nm_block_size;
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/* Map RX/TX rings. The TX ring is located after the RX ring */
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rx_ring = mmap(NULL, 2 * ring_size, PROT_READ | PROT_WRITE,
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MAP_SHARED, fd, 0);
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if ((long)rx_ring == -1L)
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exit(1);
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tx_ring = rx_ring + ring_size:
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Message reception:
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This example assumes some ring parameters of the ring setup are available.
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unsigned int frame_offset = 0;
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struct nl_mmap_hdr *hdr;
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struct nlmsghdr *nlh;
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unsigned char buf[16384];
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ssize_t len;
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while (1) {
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struct pollfd pfds[1];
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pfds[0].fd = fd;
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pfds[0].events = POLLIN | POLLERR;
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pfds[0].revents = 0;
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if (poll(pfds, 1, -1) < 0 && errno != -EINTR)
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exit(1);
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/* Check for errors. Error handling omitted */
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if (pfds[0].revents & POLLERR)
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<handle error>
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/* If no new messages, poll again */
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if (!(pfds[0].revents & POLLIN))
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continue;
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/* Process all frames */
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while (1) {
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/* Get next frame header */
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hdr = rx_ring + frame_offset;
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if (hdr->nm_status == NL_MMAP_STATUS_VALID)
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/* Regular memory mapped frame */
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nlh = (void *hdr) + NL_MMAP_HDRLEN;
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len = hdr->nm_len;
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/* Release empty message immediately. May happen
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* on error during message construction.
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*/
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if (len == 0)
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goto release;
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} else if (hdr->nm_status == NL_MMAP_STATUS_COPY) {
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/* Frame queued to socket receive queue */
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len = recv(fd, buf, sizeof(buf), MSG_DONTWAIT);
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if (len <= 0)
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break;
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nlh = buf;
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} else
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/* No more messages to process, continue polling */
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break;
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process_msg(nlh);
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release:
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/* Release frame back to the kernel */
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hdr->nm_status = NL_MMAP_STATUS_UNUSED;
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/* Advance frame offset to next frame */
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frame_offset = (frame_offset + frame_size) % ring_size;
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}
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}
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Message transmission:
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This example assumes some ring parameters of the ring setup are available.
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A single message is constructed and transmitted, to send multiple messages
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at once they would be constructed in consecutive frames before a final call
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to sendto().
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unsigned int frame_offset = 0;
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struct nl_mmap_hdr *hdr;
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struct nlmsghdr *nlh;
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struct sockaddr_nl addr = {
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.nl_family = AF_NETLINK,
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};
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hdr = tx_ring + frame_offset;
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if (hdr->nm_status != NL_MMAP_STATUS_UNUSED)
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/* No frame available. Use poll() to avoid. */
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exit(1);
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nlh = (void *)hdr + NL_MMAP_HDRLEN;
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/* Build message */
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build_message(nlh);
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/* Fill frame header: length and status need to be set */
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hdr->nm_len = nlh->nlmsg_len;
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hdr->nm_status = NL_MMAP_STATUS_VALID;
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if (sendto(fd, NULL, 0, 0, &addr, sizeof(addr)) < 0)
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exit(1);
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/* Advance frame offset to next frame */
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frame_offset = (frame_offset + frame_size) % ring_size;
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