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320 lines
11 KiB
320 lines
11 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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============================================== |
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Management Component Transport Protocol (MCTP) |
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============================================== |
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net/mctp/ contains protocol support for MCTP, as defined by DMTF standard |
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DSP0236. Physical interface drivers ("bindings" in the specification) are |
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provided in drivers/net/mctp/. |
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The core code provides a socket-based interface to send and receive MCTP |
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messages, through an AF_MCTP, SOCK_DGRAM socket. |
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Structure: interfaces & networks |
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================================ |
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The kernel models the local MCTP topology through two items: interfaces and |
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networks. |
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An interface (or "link") is an instance of an MCTP physical transport binding |
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(as defined by DSP0236, section 3.2.47), likely connected to a specific hardware |
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device. This is represented as a ``struct netdevice``. |
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A network defines a unique address space for MCTP endpoints by endpoint-ID |
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(described by DSP0236, section 3.2.31). A network has a user-visible identifier |
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to allow references from userspace. Route definitions are specific to one |
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network. |
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Interfaces are associated with one network. A network may be associated with one |
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or more interfaces. |
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If multiple networks are present, each may contain endpoint IDs (EIDs) that are |
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also present on other networks. |
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Sockets API |
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=========== |
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Protocol definitions |
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-------------------- |
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MCTP uses ``AF_MCTP`` / ``PF_MCTP`` for the address- and protocol- families. |
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Since MCTP is message-based, only ``SOCK_DGRAM`` sockets are supported. |
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.. code-block:: C |
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int sd = socket(AF_MCTP, SOCK_DGRAM, 0); |
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The only (current) value for the ``protocol`` argument is 0. |
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As with all socket address families, source and destination addresses are |
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specified with a ``sockaddr`` type, with a single-byte endpoint address: |
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.. code-block:: C |
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typedef __u8 mctp_eid_t; |
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struct mctp_addr { |
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mctp_eid_t s_addr; |
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}; |
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struct sockaddr_mctp { |
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__kernel_sa_family_t smctp_family; |
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unsigned int smctp_network; |
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struct mctp_addr smctp_addr; |
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__u8 smctp_type; |
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__u8 smctp_tag; |
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}; |
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#define MCTP_NET_ANY 0x0 |
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#define MCTP_ADDR_ANY 0xff |
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Syscall behaviour |
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----------------- |
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The following sections describe the MCTP-specific behaviours of the standard |
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socket system calls. These behaviours have been chosen to map closely to the |
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existing sockets APIs. |
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``bind()`` : set local socket address |
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
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Sockets that receive incoming request packets will bind to a local address, |
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using the ``bind()`` syscall. |
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.. code-block:: C |
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struct sockaddr_mctp addr; |
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addr.smctp_family = AF_MCTP; |
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addr.smctp_network = MCTP_NET_ANY; |
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addr.smctp_addr.s_addr = MCTP_ADDR_ANY; |
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addr.smctp_type = MCTP_TYPE_PLDM; |
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addr.smctp_tag = MCTP_TAG_OWNER; |
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int rc = bind(sd, (struct sockaddr *)&addr, sizeof(addr)); |
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This establishes the local address of the socket. Incoming MCTP messages that |
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match the network, address, and message type will be received by this socket. |
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The reference to 'incoming' is important here; a bound socket will only receive |
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messages with the TO bit set, to indicate an incoming request message, rather |
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than a response. |
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The ``smctp_tag`` value will configure the tags accepted from the remote side of |
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this socket. Given the above, the only valid value is ``MCTP_TAG_OWNER``, which |
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will result in remotely "owned" tags being routed to this socket. Since |
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``MCTP_TAG_OWNER`` is set, the 3 least-significant bits of ``smctp_tag`` are not |
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used; callers must set them to zero. |
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A ``smctp_network`` value of ``MCTP_NET_ANY`` will configure the socket to |
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receive incoming packets from any locally-connected network. A specific network |
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value will cause the socket to only receive incoming messages from that network. |
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The ``smctp_addr`` field specifies a local address to bind to. A value of |
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``MCTP_ADDR_ANY`` configures the socket to receive messages addressed to any |
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local destination EID. |
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The ``smctp_type`` field specifies which message types to receive. Only the |
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lower 7 bits of the type is matched on incoming messages (ie., the |
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most-significant IC bit is not part of the match). This results in the socket |
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receiving packets with and without a message integrity check footer. |
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``sendto()``, ``sendmsg()``, ``send()`` : transmit an MCTP message |
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
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An MCTP message is transmitted using one of the ``sendto()``, ``sendmsg()`` or |
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``send()`` syscalls. Using ``sendto()`` as the primary example: |
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.. code-block:: C |
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struct sockaddr_mctp addr; |
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char buf[14]; |
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ssize_t len; |
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/* set message destination */ |
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addr.smctp_family = AF_MCTP; |
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addr.smctp_network = 0; |
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addr.smctp_addr.s_addr = 8; |
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addr.smctp_tag = MCTP_TAG_OWNER; |
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addr.smctp_type = MCTP_TYPE_ECHO; |
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/* arbitrary message to send, with message-type header */ |
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buf[0] = MCTP_TYPE_ECHO; |
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memcpy(buf + 1, "hello, world!", sizeof(buf) - 1); |
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len = sendto(sd, buf, sizeof(buf), 0, |
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(struct sockaddr_mctp *)&addr, sizeof(addr)); |
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The network and address fields of ``addr`` define the remote address to send to. |
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If ``smctp_tag`` has the ``MCTP_TAG_OWNER``, the kernel will ignore any bits set |
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in ``MCTP_TAG_VALUE``, and generate a tag value suitable for the destination |
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EID. If ``MCTP_TAG_OWNER`` is not set, the message will be sent with the tag |
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value as specified. If a tag value cannot be allocated, the system call will |
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report an errno of ``EAGAIN``. |
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The application must provide the message type byte as the first byte of the |
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message buffer passed to ``sendto()``. If a message integrity check is to be |
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included in the transmitted message, it must also be provided in the message |
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buffer, and the most-significant bit of the message type byte must be 1. |
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The ``sendmsg()`` system call allows a more compact argument interface, and the |
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message buffer to be specified as a scatter-gather list. At present no ancillary |
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message types (used for the ``msg_control`` data passed to ``sendmsg()``) are |
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defined. |
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Transmitting a message on an unconnected socket with ``MCTP_TAG_OWNER`` |
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specified will cause an allocation of a tag, if no valid tag is already |
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allocated for that destination. The (destination-eid,tag) tuple acts as an |
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implicit local socket address, to allow the socket to receive responses to this |
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outgoing message. If any previous allocation has been performed (to for a |
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different remote EID), that allocation is lost. |
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Sockets will only receive responses to requests they have sent (with TO=1) and |
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may only respond (with TO=0) to requests they have received. |
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``recvfrom()``, ``recvmsg()``, ``recv()`` : receive an MCTP message |
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
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An MCTP message can be received by an application using one of the |
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``recvfrom()``, ``recvmsg()``, or ``recv()`` system calls. Using ``recvfrom()`` |
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as the primary example: |
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.. code-block:: C |
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struct sockaddr_mctp addr; |
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socklen_t addrlen; |
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char buf[14]; |
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ssize_t len; |
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addrlen = sizeof(addr); |
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len = recvfrom(sd, buf, sizeof(buf), 0, |
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(struct sockaddr_mctp *)&addr, &addrlen); |
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/* We can expect addr to describe an MCTP address */ |
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assert(addrlen >= sizeof(buf)); |
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assert(addr.smctp_family == AF_MCTP); |
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printf("received %zd bytes from remote EID %d\n", rc, addr.smctp_addr); |
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The address argument to ``recvfrom`` and ``recvmsg`` is populated with the |
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remote address of the incoming message, including tag value (this will be needed |
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in order to reply to the message). |
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The first byte of the message buffer will contain the message type byte. If an |
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integrity check follows the message, it will be included in the received buffer. |
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The ``recv()`` system call behaves in a similar way, but does not provide a |
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remote address to the application. Therefore, these are only useful if the |
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remote address is already known, or the message does not require a reply. |
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Like the send calls, sockets will only receive responses to requests they have |
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sent (TO=1) and may only respond (TO=0) to requests they have received. |
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``ioctl(SIOCMCTPALLOCTAG)`` and ``ioctl(SIOCMCTPDROPTAG)`` |
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
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These tags give applications more control over MCTP message tags, by allocating |
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(and dropping) tag values explicitly, rather than the kernel automatically |
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allocating a per-message tag at ``sendmsg()`` time. |
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In general, you will only need to use these ioctls if your MCTP protocol does |
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not fit the usual request/response model. For example, if you need to persist |
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tags across multiple requests, or a request may generate more than one response. |
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In these cases, the ioctls allow you to decouple the tag allocation (and |
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release) from individual message send and receive operations. |
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Both ioctls are passed a pointer to a ``struct mctp_ioc_tag_ctl``: |
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.. code-block:: C |
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struct mctp_ioc_tag_ctl { |
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mctp_eid_t peer_addr; |
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__u8 tag; |
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__u16 flags; |
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}; |
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``SIOCMCTPALLOCTAG`` allocates a tag for a specific peer, which an application |
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can use in future ``sendmsg()`` calls. The application populates the |
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``peer_addr`` member with the remote EID. Other fields must be zero. |
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On return, the ``tag`` member will be populated with the allocated tag value. |
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The allocated tag will have the following tag bits set: |
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- ``MCTP_TAG_OWNER``: it only makes sense to allocate tags if you're the tag |
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owner |
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- ``MCTP_TAG_PREALLOC``: to indicate to ``sendmsg()`` that this is a |
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preallocated tag. |
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- ... and the actual tag value, within the least-significant three bits |
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(``MCTP_TAG_MASK``). Note that zero is a valid tag value. |
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The tag value should be used as-is for the ``smctp_tag`` member of ``struct |
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sockaddr_mctp``. |
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``SIOCMCTPDROPTAG`` releases a tag that has been previously allocated by a |
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``SIOCMCTPALLOCTAG`` ioctl. The ``peer_addr`` must be the same as used for the |
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allocation, and the ``tag`` value must match exactly the tag returned from the |
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allocation (including the ``MCTP_TAG_OWNER`` and ``MCTP_TAG_PREALLOC`` bits). |
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The ``flags`` field must be zero. |
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Kernel internals |
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================ |
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There are a few possible packet flows in the MCTP stack: |
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1. local TX to remote endpoint, message <= MTU:: |
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sendmsg() |
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-> mctp_local_output() |
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: route lookup |
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-> rt->output() (== mctp_route_output) |
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-> dev_queue_xmit() |
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2. local TX to remote endpoint, message > MTU:: |
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sendmsg() |
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-> mctp_local_output() |
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-> mctp_do_fragment_route() |
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: creates packet-sized skbs. For each new skb: |
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-> rt->output() (== mctp_route_output) |
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-> dev_queue_xmit() |
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3. remote TX to local endpoint, single-packet message:: |
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mctp_pkttype_receive() |
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: route lookup |
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-> rt->output() (== mctp_route_input) |
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: sk_key lookup |
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-> sock_queue_rcv_skb() |
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4. remote TX to local endpoint, multiple-packet message:: |
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mctp_pkttype_receive() |
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: route lookup |
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-> rt->output() (== mctp_route_input) |
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: sk_key lookup |
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: stores skb in struct sk_key->reasm_head |
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mctp_pkttype_receive() |
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: route lookup |
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-> rt->output() (== mctp_route_input) |
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: sk_key lookup |
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: finds existing reassembly in sk_key->reasm_head |
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: appends new fragment |
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-> sock_queue_rcv_skb() |
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Key refcounts |
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------------- |
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* keys are refed by: |
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- a skb: during route output, stored in ``skb->cb``. |
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- netns and sock lists. |
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* keys can be associated with a device, in which case they hold a |
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reference to the dev (set through ``key->dev``, counted through |
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``dev->key_count``). Multiple keys can reference the device.
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