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1.. SPDX-License-Identifier: GPL-2.02 3==============================================4Management Component Transport Protocol (MCTP)5==============================================6 7net/mctp/ contains protocol support for MCTP, as defined by DMTF standard8DSP0236. Physical interface drivers ("bindings" in the specification) are9provided in drivers/net/mctp/.10 11The core code provides a socket-based interface to send and receive MCTP12messages, through an AF_MCTP, SOCK_DGRAM socket.13 14Structure: interfaces & networks15================================16 17The kernel models the local MCTP topology through two items: interfaces and18networks.19 20An interface (or "link") is an instance of an MCTP physical transport binding21(as defined by DSP0236, section 3.2.47), likely connected to a specific hardware22device. This is represented as a ``struct netdevice``.23 24A network defines a unique address space for MCTP endpoints by endpoint-ID25(described by DSP0236, section 3.2.31). A network has a user-visible identifier26to allow references from userspace. Route definitions are specific to one27network.28 29Interfaces are associated with one network. A network may be associated with one30or more interfaces.31 32If multiple networks are present, each may contain endpoint IDs (EIDs) that are33also present on other networks.34 35Sockets API36===========37 38Protocol definitions39--------------------40 41MCTP uses ``AF_MCTP`` / ``PF_MCTP`` for the address- and protocol- families.42Since MCTP is message-based, only ``SOCK_DGRAM`` sockets are supported.43 44.. code-block:: C45 46 int sd = socket(AF_MCTP, SOCK_DGRAM, 0);47 48The only (current) value for the ``protocol`` argument is 0.49 50As with all socket address families, source and destination addresses are51specified with a ``sockaddr`` type, with a single-byte endpoint address:52 53.. code-block:: C54 55 typedef __u8 mctp_eid_t;56 57 struct mctp_addr {58 mctp_eid_t s_addr;59 };60 61 struct sockaddr_mctp {62 __kernel_sa_family_t smctp_family;63 unsigned int smctp_network;64 struct mctp_addr smctp_addr;65 __u8 smctp_type;66 __u8 smctp_tag;67 };68 69 #define MCTP_NET_ANY 0x070 #define MCTP_ADDR_ANY 0xff71 72 73Syscall behaviour74-----------------75 76The following sections describe the MCTP-specific behaviours of the standard77socket system calls. These behaviours have been chosen to map closely to the78existing sockets APIs.79 80``bind()`` : set local socket address81^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^82 83Sockets that receive incoming request packets will bind to a local address,84using the ``bind()`` syscall.85 86.. code-block:: C87 88 struct sockaddr_mctp addr;89 90 addr.smctp_family = AF_MCTP;91 addr.smctp_network = MCTP_NET_ANY;92 addr.smctp_addr.s_addr = MCTP_ADDR_ANY;93 addr.smctp_type = MCTP_TYPE_PLDM;94 addr.smctp_tag = MCTP_TAG_OWNER;95 96 int rc = bind(sd, (struct sockaddr *)&addr, sizeof(addr));97 98This establishes the local address of the socket. Incoming MCTP messages that99match the network, address, and message type will be received by this socket.100The reference to 'incoming' is important here; a bound socket will only receive101messages with the TO bit set, to indicate an incoming request message, rather102than a response.103 104The ``smctp_tag`` value will configure the tags accepted from the remote side of105this socket. Given the above, the only valid value is ``MCTP_TAG_OWNER``, which106will result in remotely "owned" tags being routed to this socket. Since107``MCTP_TAG_OWNER`` is set, the 3 least-significant bits of ``smctp_tag`` are not108used; callers must set them to zero.109 110A ``smctp_network`` value of ``MCTP_NET_ANY`` will configure the socket to111receive incoming packets from any locally-connected network. A specific network112value will cause the socket to only receive incoming messages from that network.113 114The ``smctp_addr`` field specifies a local address to bind to. A value of115``MCTP_ADDR_ANY`` configures the socket to receive messages addressed to any116local destination EID.117 118The ``smctp_type`` field specifies which message types to receive. Only the119lower 7 bits of the type is matched on incoming messages (ie., the120most-significant IC bit is not part of the match). This results in the socket121receiving packets with and without a message integrity check footer.122 123``sendto()``, ``sendmsg()``, ``send()`` : transmit an MCTP message124^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^125 126An MCTP message is transmitted using one of the ``sendto()``, ``sendmsg()`` or127``send()`` syscalls. Using ``sendto()`` as the primary example:128 129.. code-block:: C130 131 struct sockaddr_mctp addr;132 char buf[14];133 ssize_t len;134 135 /* set message destination */136 addr.smctp_family = AF_MCTP;137 addr.smctp_network = 0;138 addr.smctp_addr.s_addr = 8;139 addr.smctp_tag = MCTP_TAG_OWNER;140 addr.smctp_type = MCTP_TYPE_ECHO;141 142 /* arbitrary message to send, with message-type header */143 buf[0] = MCTP_TYPE_ECHO;144 memcpy(buf + 1, "hello, world!", sizeof(buf) - 1);145 146 len = sendto(sd, buf, sizeof(buf), 0,147 (struct sockaddr_mctp *)&addr, sizeof(addr));148 149The network and address fields of ``addr`` define the remote address to send to.150If ``smctp_tag`` has the ``MCTP_TAG_OWNER``, the kernel will ignore any bits set151in ``MCTP_TAG_VALUE``, and generate a tag value suitable for the destination152EID. If ``MCTP_TAG_OWNER`` is not set, the message will be sent with the tag153value as specified. If a tag value cannot be allocated, the system call will154report an errno of ``EAGAIN``.155 156The application must provide the message type byte as the first byte of the157message buffer passed to ``sendto()``. If a message integrity check is to be158included in the transmitted message, it must also be provided in the message159buffer, and the most-significant bit of the message type byte must be 1.160 161The ``sendmsg()`` system call allows a more compact argument interface, and the162message buffer to be specified as a scatter-gather list. At present no ancillary163message types (used for the ``msg_control`` data passed to ``sendmsg()``) are164defined.165 166Transmitting a message on an unconnected socket with ``MCTP_TAG_OWNER``167specified will cause an allocation of a tag, if no valid tag is already168allocated for that destination. The (destination-eid,tag) tuple acts as an169implicit local socket address, to allow the socket to receive responses to this170outgoing message. If any previous allocation has been performed (to for a171different remote EID), that allocation is lost.172 173Sockets will only receive responses to requests they have sent (with TO=1) and174may only respond (with TO=0) to requests they have received.175 176``recvfrom()``, ``recvmsg()``, ``recv()`` : receive an MCTP message177^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^178 179An MCTP message can be received by an application using one of the180``recvfrom()``, ``recvmsg()``, or ``recv()`` system calls. Using ``recvfrom()``181as the primary example:182 183.. code-block:: C184 185 struct sockaddr_mctp addr;186 socklen_t addrlen;187 char buf[14];188 ssize_t len;189 190 addrlen = sizeof(addr);191 192 len = recvfrom(sd, buf, sizeof(buf), 0,193 (struct sockaddr_mctp *)&addr, &addrlen);194 195 /* We can expect addr to describe an MCTP address */196 assert(addrlen >= sizeof(buf));197 assert(addr.smctp_family == AF_MCTP);198 199 printf("received %zd bytes from remote EID %d\n", rc, addr.smctp_addr);200 201The address argument to ``recvfrom`` and ``recvmsg`` is populated with the202remote address of the incoming message, including tag value (this will be needed203in order to reply to the message).204 205The first byte of the message buffer will contain the message type byte. If an206integrity check follows the message, it will be included in the received buffer.207 208The ``recv()`` system call behaves in a similar way, but does not provide a209remote address to the application. Therefore, these are only useful if the210remote address is already known, or the message does not require a reply.211 212Like the send calls, sockets will only receive responses to requests they have213sent (TO=1) and may only respond (TO=0) to requests they have received.214 215``ioctl(SIOCMCTPALLOCTAG)`` and ``ioctl(SIOCMCTPDROPTAG)``216^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^217 218These tags give applications more control over MCTP message tags, by allocating219(and dropping) tag values explicitly, rather than the kernel automatically220allocating a per-message tag at ``sendmsg()`` time.221 222In general, you will only need to use these ioctls if your MCTP protocol does223not fit the usual request/response model. For example, if you need to persist224tags across multiple requests, or a request may generate more than one response.225In these cases, the ioctls allow you to decouple the tag allocation (and226release) from individual message send and receive operations.227 228Both ioctls are passed a pointer to a ``struct mctp_ioc_tag_ctl``:229 230.. code-block:: C231 232 struct mctp_ioc_tag_ctl {233 mctp_eid_t peer_addr;234 __u8 tag;235 __u16 flags;236 };237 238``SIOCMCTPALLOCTAG`` allocates a tag for a specific peer, which an application239can use in future ``sendmsg()`` calls. The application populates the240``peer_addr`` member with the remote EID. Other fields must be zero.241 242On return, the ``tag`` member will be populated with the allocated tag value.243The allocated tag will have the following tag bits set:244 245 - ``MCTP_TAG_OWNER``: it only makes sense to allocate tags if you're the tag246 owner247 248 - ``MCTP_TAG_PREALLOC``: to indicate to ``sendmsg()`` that this is a249 preallocated tag.250 251 - ... and the actual tag value, within the least-significant three bits252 (``MCTP_TAG_MASK``). Note that zero is a valid tag value.253 254The tag value should be used as-is for the ``smctp_tag`` member of ``struct255sockaddr_mctp``.256 257``SIOCMCTPDROPTAG`` releases a tag that has been previously allocated by a258``SIOCMCTPALLOCTAG`` ioctl. The ``peer_addr`` must be the same as used for the259allocation, and the ``tag`` value must match exactly the tag returned from the260allocation (including the ``MCTP_TAG_OWNER`` and ``MCTP_TAG_PREALLOC`` bits).261The ``flags`` field must be zero.262 263Kernel internals264================265 266There are a few possible packet flows in the MCTP stack:267 2681. local TX to remote endpoint, message <= MTU::269 270 sendmsg()271 -> mctp_local_output()272 : route lookup273 -> rt->output() (== mctp_route_output)274 -> dev_queue_xmit()275 2762. local TX to remote endpoint, message > MTU::277 278 sendmsg()279 -> mctp_local_output()280 -> mctp_do_fragment_route()281 : creates packet-sized skbs. For each new skb:282 -> rt->output() (== mctp_route_output)283 -> dev_queue_xmit()284 2853. remote TX to local endpoint, single-packet message::286 287 mctp_pkttype_receive()288 : route lookup289 -> rt->output() (== mctp_route_input)290 : sk_key lookup291 -> sock_queue_rcv_skb()292 2934. remote TX to local endpoint, multiple-packet message::294 295 mctp_pkttype_receive()296 : route lookup297 -> rt->output() (== mctp_route_input)298 : sk_key lookup299 : stores skb in struct sk_key->reasm_head300 301 mctp_pkttype_receive()302 : route lookup303 -> rt->output() (== mctp_route_input)304 : sk_key lookup305 : finds existing reassembly in sk_key->reasm_head306 : appends new fragment307 -> sock_queue_rcv_skb()308 309Key refcounts310-------------311 312 * keys are refed by:313 314 - a skb: during route output, stored in ``skb->cb``.315 316 - netns and sock lists.317 318 * keys can be associated with a device, in which case they hold a319 reference to the dev (set through ``key->dev``, counted through320 ``dev->key_count``). Multiple keys can reference the device.321