docs: convert vpp doc md->rst

Type: improvement

Change-Id: If453321785b04f9c16e8cea36fb1910efaeb2c59
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>

1 files changed, 342 insertions, 0 deletions

diff --git a/src/vlibapi/api_doc.rst b/src/vlibapi/api_doc.rst
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+.. _api_doc:
+
+Writing API handlers
+====================
+
+VPP provides a binary API scheme to allow a wide variety of client codes
+to program data-plane tables. As of this writing, there are hundreds of
+binary APIs.
+
+Messages are defined in ``*.api`` files. Today, there are about 50 api
+files, with more arriving as folks add programmable features. The API
+file compiler sources reside in @ref src/tools/vppapigen.
+
+From @ref src/vnet/interface.api, here’s a typical request/response
+message definition:
+
+.. code:: c
+
+        autoreply define sw_interface_set_flags
+        {
+          u32 client_index;
+          u32 context;
+          u32 sw_if_index;
+          /* 1 = up, 0 = down */
+          u8 admin_up_down;
+        };
+
+To a first approximation, the API compiler renders this definition into
+``build-root/.../vpp/include/vnet/interface.api.h`` as follows:
+
+.. code:: c
+
+       /****** Message ID / handler enum ******/
+       #ifdef vl_msg_id
+       vl_msg_id(VL_API_SW_INTERFACE_SET_FLAGS, vl_api_sw_interface_set_flags_t_handler)
+       vl_msg_id(VL_API_SW_INTERFACE_SET_FLAGS_REPLY, vl_api_sw_interface_set_flags_reply_t_handler)
+       #endif
+
+       /****** Message names ******/
+       #ifdef vl_msg_name
+       vl_msg_name(vl_api_sw_interface_set_flags_t, 1)
+       vl_msg_name(vl_api_sw_interface_set_flags_reply_t, 1)
+       #endif
+
+       /****** Message name, crc list ******/
+       #ifdef vl_msg_name_crc_list
+       #define foreach_vl_msg_name_crc_interface \
+       _(VL_API_SW_INTERFACE_SET_FLAGS, sw_interface_set_flags, f890584a) \
+       _(VL_API_SW_INTERFACE_SET_FLAGS_REPLY, sw_interface_set_flags_reply, dfbf3afa) \
+       #endif
+
+       /****** Typedefs *****/
+       #ifdef vl_typedefs
+       typedef VL_API_PACKED(struct _vl_api_sw_interface_set_flags {
+           u16 _vl_msg_id;
+           u32 client_index;
+           u32 context;
+           u32 sw_if_index;
+           u8 admin_up_down;
+       }) vl_api_sw_interface_set_flags_t;
+
+       typedef VL_API_PACKED(struct _vl_api_sw_interface_set_flags_reply {
+           u16 _vl_msg_id;
+           u32 context;
+           i32 retval;
+       }) vl_api_sw_interface_set_flags_reply_t;
+
+       ...
+       #endif /* vl_typedefs */
+
+To change the admin state of an interface, a binary api client sends a
+@ref vl_api_sw_interface_set_flags_t to VPP, which will respond with a
+@ref vl_api_sw_interface_set_flags_reply_t message.
+
+Multiple layers of software, transport types, and shared libraries
+implement a variety of features:
+
+-  API message allocation, tracing, pretty-printing, and replay.
+-  Message transport via global shared memory, pairwise/private shared
+   memory, and sockets.
+-  Barrier synchronization of worker threads across thread-unsafe
+   message handlers.
+
+Correctly-coded message handlers know nothing about the transport used
+to deliver messages to/from VPP. It’s reasonably straightforward to use
+multiple API message transport types simultaneously.
+
+For historical reasons, binary api messages are (putatively) sent in
+network byte order. As of this writing, we’re seriously considering
+whether that choice makes sense.
+
+Message Allocation
+------------------
+
+Since binary API messages are always processed in order, we allocate
+messages using a ring allocator whenever possible. This scheme is
+extremely fast when compared with a traditional memory allocator, and
+doesn’t cause heap fragmentation. See @ref
+src/vlibmemory/memory_shared.c @ref vl_msg_api_alloc_internal().
+
+Regardless of transport, binary api messages always follow a @ref
+msgbuf_t header:
+
+.. code:: c
+
+       typedef struct msgbuf_
+       {
+         unix_shared_memory_queue_t *q;
+         u32 data_len;
+         u32 gc_mark_timestamp;
+         u8 data[0];
+       } msgbuf_t;
+
+This structure makes it easy to trace messages without having to decode
+them - simply save data_len bytes - and allows @ref vl_msg_api_free() to
+rapidly dispose of message buffers:
+
+.. code:: c
+
+       void
+       vl_msg_api_free (void *a)
+       {
+         msgbuf_t *rv;
+         api_main_t *am = &api_main;
+
+         rv = (msgbuf_t *) (((u8 *) a) - offsetof (msgbuf_t, data));
+
+         /*
+          * Here's the beauty of the scheme.  Only one proc/thread has
+          * control of a given message buffer. To free a buffer, we just
+          * clear the queue field, and leave. No locks, no hits, no errors...
+          */
+         if (rv->q)
+           {
+             rv->q = 0;
+             rv->gc_mark_timestamp = 0;
+             return;
+           }
+         <snip>
+       }
+
+Message Tracing and Replay
+--------------------------
+
+It’s extremely important that VPP can capture and replay sizeable binary
+API traces. System-level issues involving hundreds of thousands of API
+transactions can be re-run in a second or less. Partial replay allows
+one to binary-search for the point where the wheels fall off. One can
+add scaffolding to the data plane, to trigger when complex conditions
+obtain.
+
+With binary API trace, print, and replay, system-level bug reports of
+the form “after 300,000 API transactions, the VPP data-plane stopped
+forwarding traffic, FIX IT!” can be solved offline.
+
+More often than not, one discovers that a control-plane client
+misprograms the data plane after a long time or under complex
+circumstances. Without direct evidence, “it’s a data-plane problem!”
+
+See @ref src/vlibmemory/memory_vlib.c @ref vl_msg_api_process_file(),
+and @ref src/vlibapi/api_shared.c. See also the debug CLI command “api
+trace”
+
+Client connection details
+-------------------------
+
+Establishing a binary API connection to VPP from a C-language client is
+easy:
+
+.. code:: c
+
+           int
+           connect_to_vpe (char *client_name, int client_message_queue_length)
+           {
+             vat_main_t *vam = &vat_main;
+             api_main_t *am = &api_main;
+
+             if (vl_client_connect_to_vlib ("/vpe-api", client_name,
+                                           client_message_queue_length) < 0)
+               return -1;
+
+             /* Memorize vpp's binary API message input queue address */
+             vam->vl_input_queue = am->shmem_hdr->vl_input_queue;
+             /* And our client index */
+             vam->my_client_index = am->my_client_index;
+             return 0;
+           }
+
+32 is a typical value for client_message_queue_length. VPP cannot block
+when it needs to send an API message to a binary API client, and the
+VPP-side binary API message handlers are very fast. When sending
+asynchronous messages, make sure to scrape the binary API rx ring with
+some enthusiasm.
+
+binary API message RX pthread
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Calling @ref vl_client_connect_to_vlib spins up a binary API message RX
+pthread:
+
+.. code:: c
+
+           static void *
+           rx_thread_fn (void *arg)
+           {
+             unix_shared_memory_queue_t *q;
+             memory_client_main_t *mm = &memory_client_main;
+             api_main_t *am = &api_main;
+
+             q = am->vl_input_queue;
+
+             /* So we can make the rx thread terminate cleanly */
+             if (setjmp (mm->rx_thread_jmpbuf) == 0)
+               {
+                 mm->rx_thread_jmpbuf_valid = 1;
+                 while (1)
+               {
+                 vl_msg_api_queue_handler (q);
+               }
+               }
+             pthread_exit (0);
+           }
+
+To handle the binary API message queue yourself, use @ref
+vl_client_connect_to_vlib_no_rx_pthread.
+
+In turn, vl_msg_api_queue_handler(…) uses mutex/condvar signalling to
+wake up, process VPP -> client traffic, then sleep. VPP supplies a
+condvar broadcast when the VPP -> client API message queue transitions
+from empty to nonempty.
+
+VPP checks its own binary API input queue at a very high rate. VPP
+invokes message handlers in “process” context [aka cooperative
+multitasking thread context] at a variable rate, depending on data-plane
+packet processing requirements.
+
+Client disconnection details
+----------------------------
+
+To disconnect from VPP, call @ref vl_client_disconnect_from_vlib. Please
+arrange to call this function if the client application terminates
+abnormally. VPP makes every effort to hold a decent funeral for dead
+clients, but VPP can’t guarantee to free leaked memory in the shared
+binary API segment.
+
+Sending binary API messages to VPP
+----------------------------------
+
+The point of the exercise is to send binary API messages to VPP, and to
+receive replies from VPP. Many VPP binary APIs comprise a client request
+message, and a simple status reply. For example, to set the admin status
+of an interface, one codes:
+
+.. code:: c
+
+       vl_api_sw_interface_set_flags_t *mp;
+
+       mp = vl_msg_api_alloc (sizeof (*mp));
+       memset (mp, 0, sizeof (*mp));
+       mp->_vl_msg_id = clib_host_to_net_u16 (VL_API_SW_INTERFACE_SET_FLAGS);
+       mp->client_index = api_main.my_client_index;
+       mp->sw_if_index = clib_host_to_net_u32 (<interface-sw-if-index>);
+       vl_msg_api_send (api_main.shmem_hdr->vl_input_queue, (u8 *)mp);
+
+Key points:
+
+-  Use @ref vl_msg_api_alloc to allocate message buffers
+
+-  Allocated message buffers are not initialized, and must be presumed
+   to contain trash.
+
+-  Don’t forget to set the \_vl_msg_id field!
+
+-  As of this writing, binary API message IDs and data are sent in
+   network byte order
+
+-  The client-library global data structure @ref api_main keeps track of
+   sufficient pointers and handles used to communicate with VPP
+
+Receiving binary API messages from VPP
+--------------------------------------
+
+Unless you’ve made other arrangements (see @ref
+vl_client_connect_to_vlib_no_rx_pthread), *messages are received on a
+separate rx pthread*. Synchronization with the client application main
+thread is the responsibility of the application!
+
+Set up message handlers about as follows:
+
+.. code:: c
+
+       #define vl_typedefs     /* define message structures */
+       #include <vpp/api/vpe_all_api_h.h>
+       #undef vl_typedefs
+
+       /* declare message handlers for each api */
+
+       #define vl_endianfun        /* define message structures */
+       #include <vpp/api/vpe_all_api_h.h>
+       #undef vl_endianfun
+
+       /* instantiate all the print functions we know about */
+       #define vl_print(handle, ...)
+       #define vl_printfun
+       #include <vpp/api/vpe_all_api_h.h>
+       #undef vl_printfun
+
+       /* Define a list of all message that the client handles */
+       #define foreach_vpe_api_reply_msg                            \
+          _(SW_INTERFACE_SET_FLAGS_REPLY, sw_interface_set_flags_reply)
+
+          static clib_error_t *
+          my_api_hookup (vlib_main_t * vm)
+          {
+            api_main_t *am = &api_main;
+
+          #define _(N,n)                                                  \
+              vl_msg_api_set_handlers(VL_API_##N, #n,                     \
+                                     vl_api_##n##_t_handler,              \
+                                     vl_noop_handler,                     \
+                                     vl_api_##n##_t_endian,               \
+                                     vl_api_##n##_t_print,                \
+                                     sizeof(vl_api_##n##_t), 1);
+            foreach_vpe_api_msg;
+          #undef _
+
+            return 0;
+           }
+
+The key API used to establish message handlers is @ref
+vl_msg_api_set_handlers , which sets values in multiple parallel vectors
+in the @ref api_main_t structure. As of this writing: not all vector
+element values can be set through the API. You’ll see sporadic API
+message registrations followed by minor adjustments of this form:
+
+.. code:: c
+
+       /*
+        * Thread-safe API messages
+        */
+       am->is_mp_safe[VL_API_IP_ADD_DEL_ROUTE] = 1;
+       am->is_mp_safe[VL_API_GET_NODE_GRAPH] = 1;