1.. SPDX-License-Identifier: GPL-2.0 2 3============================================== 4Management Component Transport Protocol (MCTP) 5============================================== 6 7net/mctp/ contains protocol support for MCTP, as defined by DMTF standard 8DSP0236. Physical interface drivers ("bindings" in the specification) are 9provided in drivers/net/mctp/. 10 11The core code provides a socket-based interface to send and receive MCTP 12messages, through an AF_MCTP, SOCK_DGRAM socket. 13 14Structure: interfaces & networks 15================================ 16 17The kernel models the local MCTP topology through two items: interfaces and 18networks. 19 20An interface (or "link") is an instance of an MCTP physical transport binding 21(as defined by DSP0236, section 3.2.47), likely connected to a specific hardware 22device. This is represented as a ``struct netdevice``. 23 24A network defines a unique address space for MCTP endpoints by endpoint-ID 25(described by DSP0236, section 3.2.31). A network has a user-visible identifier 26to allow references from userspace. Route definitions are specific to one 27network. 28 29Interfaces are associated with one network. A network may be associated with one 30or more interfaces. 31 32If multiple networks are present, each may contain endpoint IDs (EIDs) that are 33also present on other networks. 34 35Sockets API 36=========== 37 38Protocol definitions 39-------------------- 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:: C 45 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 are 51specified with a ``sockaddr`` type, with a single-byte endpoint address: 52 53.. code-block:: C 54 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 0x0 70 #define MCTP_ADDR_ANY 0xff 71 72 73Syscall behaviour 74----------------- 75 76The following sections describe the MCTP-specific behaviours of the standard 77socket system calls. These behaviours have been chosen to map closely to the 78existing sockets APIs. 79 80``bind()`` : set local socket address 81^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 82 83Sockets that receive incoming request packets will bind to a local address, 84using the ``bind()`` syscall. 85 86.. code-block:: C 87 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 that 99match the network, address, and message type will be received by this socket. 100The reference to 'incoming' is important here; a bound socket will only receive 101messages with the TO bit set, to indicate an incoming request message, rather 102than a response. 103 104The ``smctp_tag`` value will configure the tags accepted from the remote side of 105this socket. Given the above, the only valid value is ``MCTP_TAG_OWNER``, which 106will result in remotely "owned" tags being routed to this socket. Since 107``MCTP_TAG_OWNER`` is set, the 3 least-significant bits of ``smctp_tag`` are not 108used; callers must set them to zero. 109 110A ``smctp_network`` value of ``MCTP_NET_ANY`` will configure the socket to 111receive incoming packets from any locally-connected network. A specific network 112value 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 of 115``MCTP_ADDR_ANY`` configures the socket to receive messages addressed to any 116local destination EID. 117 118The ``smctp_type`` field specifies which message types to receive. Only the 119lower 7 bits of the type is matched on incoming messages (ie., the 120most-significant IC bit is not part of the match). This results in the socket 121receiving packets with and without a message integrity check footer. 122 123``sendto()``, ``sendmsg()``, ``send()`` : transmit an MCTP message 124^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 125 126An MCTP message is transmitted using one of the ``sendto()``, ``sendmsg()`` or 127``send()`` syscalls. Using ``sendto()`` as the primary example: 128 129.. code-block:: C 130 131 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 set 151in ``MCTP_TAG_VALUE``, and generate a tag value suitable for the destination 152EID. If ``MCTP_TAG_OWNER`` is not set, the message will be sent with the tag 153value as specified. If a tag value cannot be allocated, the system call will 154report an errno of ``EAGAIN``. 155 156The application must provide the message type byte as the first byte of the 157message buffer passed to ``sendto()``. If a message integrity check is to be 158included in the transmitted message, it must also be provided in the message 159buffer, 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 the 162message buffer to be specified as a scatter-gather list. At present no ancillary 163message types (used for the ``msg_control`` data passed to ``sendmsg()``) are 164defined. 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 already 168allocated for that destination. The (destination-eid,tag) tuple acts as an 169implicit local socket address, to allow the socket to receive responses to this 170outgoing message. If any previous allocation has been performed (to for a 171different remote EID), that allocation is lost. 172 173Sockets will only receive responses to requests they have sent (with TO=1) and 174may only respond (with TO=0) to requests they have received. 175 176``recvfrom()``, ``recvmsg()``, ``recv()`` : receive an MCTP message 177^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 178 179An MCTP message can be received by an application using one of the 180``recvfrom()``, ``recvmsg()``, or ``recv()`` system calls. Using ``recvfrom()`` 181as the primary example: 182 183.. code-block:: C 184 185 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 the 202remote address of the incoming message, including tag value (this will be needed 203in order to reply to the message). 204 205The first byte of the message buffer will contain the message type byte. If an 206integrity 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 a 209remote address to the application. Therefore, these are only useful if the 210remote 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 have 213sent (TO=1) and may only respond (TO=0) to requests they have received. 214 215Kernel internals 216================ 217 218There are a few possible packet flows in the MCTP stack: 219 2201. local TX to remote endpoint, message <= MTU:: 221 222 sendmsg() 223 -> mctp_local_output() 224 : route lookup 225 -> rt->output() (== mctp_route_output) 226 -> dev_queue_xmit() 227 2282. local TX to remote endpoint, message > MTU:: 229 230 sendmsg() 231 -> mctp_local_output() 232 -> mctp_do_fragment_route() 233 : creates packet-sized skbs. For each new skb: 234 -> rt->output() (== mctp_route_output) 235 -> dev_queue_xmit() 236 2373. remote TX to local endpoint, single-packet message:: 238 239 mctp_pkttype_receive() 240 : route lookup 241 -> rt->output() (== mctp_route_input) 242 : sk_key lookup 243 -> sock_queue_rcv_skb() 244 2454. remote TX to local endpoint, multiple-packet message:: 246 247 mctp_pkttype_receive() 248 : route lookup 249 -> rt->output() (== mctp_route_input) 250 : sk_key lookup 251 : stores skb in struct sk_key->reasm_head 252 253 mctp_pkttype_receive() 254 : route lookup 255 -> rt->output() (== mctp_route_input) 256 : sk_key lookup 257 : finds existing reassembly in sk_key->reasm_head 258 : appends new fragment 259 -> sock_queue_rcv_skb() 260 261Key refcounts 262------------- 263 264 * keys are refed by: 265 266 - a skb: during route output, stored in ``skb->cb``. 267 268 - netns and sock lists. 269 270 * keys can be associated with a device, in which case they hold a 271 reference to the dev (set through ``key->dev``, counted through 272 ``dev->key_count``). Multiple keys can reference the device. 273