Documentation: Add devlink-trap documentation

Add initial documentation of the devlink-trap mechanism, explaining the
background, motivation and the semantics of the interface.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/devlink-trap.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+============
+Devlink Trap
+============
+
+Background
+==========
+
+Devices capable of offloading the kernel's datapath and perform functions such
+as bridging and routing must also be able to send specific packets to the
+kernel (i.e., the CPU) for processing.
+
+For example, a device acting as a multicast-aware bridge must be able to send
+IGMP membership reports to the kernel for processing by the bridge module.
+Without processing such packets, the bridge module could never populate its
+MDB.
+
+As another example, consider a device acting as router which has received an IP
+packet with a TTL of 1. Upon routing the packet the device must send it to the
+kernel so that it will route it as well and generate an ICMP Time Exceeded
+error datagram. Without letting the kernel route such packets itself, utilities
+such as ``traceroute`` could never work.
+
+The fundamental ability of sending certain packets to the kernel for processing
+is called "packet trapping".
+
+Overview
+========
+
+The ``devlink-trap`` mechanism allows capable device drivers to register their
+supported packet traps with ``devlink`` and report trapped packets to
+``devlink`` for further analysis.
+
+Upon receiving trapped packets, ``devlink`` will perform a per-trap packets and
+bytes accounting and potentially report the packet to user space via a netlink
+event along with all the provided metadata (e.g., trap reason, timestamp, input
+port). This is especially useful for drop traps (see :ref:`Trap-Types`)
+as it allows users to obtain further visibility into packet drops that would
+otherwise be invisible.
+
+The following diagram provides a general overview of ``devlink-trap``::
+
+                                    Netlink event: Packet w/ metadata
+                                                   Or a summary of recent drops
+                                  ^
+                                  |
+         Userspace                |
+        +---------------------------------------------------+
+         Kernel                   |
+                                  |
+                          +-------+--------+
+                          |                |
+                          |  drop_monitor  |
+                          |                |
+                          +-------^--------+
+                                  |
+                                  |
+                                  |
+                             +----+----+
+                             |         |      Kernel's Rx path
+                             | devlink |      (non-drop traps)
+                             |         |
+                             +----^----+      ^
+                                  |           |
+                                  +-----------+
+                                  |
+                          +-------+-------+
+                          |               |
+                          | Device driver |
+                          |               |
+                          +-------^-------+
+         Kernel                   |
+        +---------------------------------------------------+
+         Hardware                 |
+                                  | Trapped packet
+                                  |
+                               +--+---+
+                               |      |
+                               | ASIC |
+                               |      |
+                               +------+
+
+.. _Trap-Types:
+
+Trap Types
+==========
+
+The ``devlink-trap`` mechanism supports the following packet trap types:
+
+  * ``drop``: Trapped packets were dropped by the underlying device. Packets
+    are only processed by ``devlink`` and not injected to the kernel's Rx path.
+    The trap action (see :ref:`Trap-Actions`) can be changed.
+  * ``exception``: Trapped packets were not forwarded as intended by the
+    underlying device due to an exception (e.g., TTL error, missing neighbour
+    entry) and trapped to the control plane for resolution. Packets are
+    processed by ``devlink`` and injected to the kernel's Rx path. Changing the
+    action of such traps is not allowed, as it can easily break the control
+    plane.
+
+.. _Trap-Actions:
+
+Trap Actions
+============
+
+The ``devlink-trap`` mechanism supports the following packet trap actions:
+
+  * ``trap``: The sole copy of the packet is sent to the CPU.
+  * ``drop``: The packet is dropped by the underlying device and a copy is not
+    sent to the CPU.
+
+Generic Packet Traps
+====================
+
+Generic packet traps are used to describe traps that trap well-defined packets
+or packets that are trapped due to well-defined conditions (e.g., TTL error).
+Such traps can be shared by multiple device drivers and their description must
+be added to the following table:
+
+.. list-table:: List of Generic Packet Traps
+   :widths: 5 5 90
+
+   * - Name
+     - Type
+     - Description
+   * - ``source_mac_is_multicast``
+     - ``drop``
+     - Traps incoming packets that the device decided to drop because of a
+       multicast source MAC
+   * - ``vlan_tag_mismatch``
+     - ``drop``
+     - Traps incoming packets that the device decided to drop in case of VLAN
+       tag mismatch: The ingress bridge port is not configured with a PVID and
+       the packet is untagged or prio-tagged
+   * - ``ingress_vlan_filter``
+     - ``drop``
+     - Traps incoming packets that the device decided to drop in case they are
+       tagged with a VLAN that is not configured on the ingress bridge port
+   * - ``ingress_spanning_tree_filter``
+     - ``drop``
+     - Traps incoming packets that the device decided to drop in case the STP
+       state of the ingress bridge port is not "forwarding"
+   * - ``port_list_is_empty``
+     - ``drop``
+     - Traps packets that the device decided to drop in case they need to be
+       flooded and the flood list is empty
+   * - ``port_loopback_filter``
+     - ``drop``
+     - Traps packets that the device decided to drop in case after layer 2
+       forwarding the only port from which they should be transmitted through
+       is the port from which they were received
+   * - ``blackhole_route``
+     - ``drop``
+     - Traps packets that the device decided to drop in case they hit a
+       blackhole route
+   * - ``ttl_value_is_too_small``
+     - ``exception``
+     - Traps unicast packets that should be forwarded by the device whose TTL
+       was decremented to 0 or less
+   * - ``tail_drop``
+     - ``drop``
+     - Traps packets that the device decided to drop because they could not be
+       enqueued to a transmission queue which is full
+
+Generic Packet Trap Groups
+==========================
+
+Generic packet trap groups are used to aggregate logically related packet
+traps. These groups allow the user to batch operations such as setting the trap
+action of all member traps. In addition, ``devlink-trap`` can report aggregated
+per-group packets and bytes statistics, in case per-trap statistics are too
+narrow. The description of these groups must be added to the following table:
+
+.. list-table:: List of Generic Packet Trap Groups
+   :widths: 10 90
+
+   * - Name
+     - Description
+   * - ``l2_drops``
+     - Contains packet traps for packets that were dropped by the device during
+       layer 2 forwarding (i.e., bridge)
+   * - ``l3_drops``
+     - Contains packet traps for packets that were dropped by the device or hit
+       an exception (e.g., TTL error) during layer 3 forwarding
+   * - ``buffer_drops``
+     - Contains packet traps for packets that were dropped by the device due to
+       an enqueue decision

Documentation/networking/index.rst

@@ -14,6 +14,7 @@ Contents:
    device_drivers/index
    dsa/index
    devlink-info-versions
+   devlink-trap
    ieee802154
    kapi
    z8530book

include/net/devlink.h

@@ -540,6 +540,9 @@ struct devlink_trap {
 	u32 metadata_cap;
 };
 
+/* All traps must be documented in
+ * Documentation/networking/devlink-trap.rst
+ */
 enum devlink_trap_generic_id {
 	DEVLINK_TRAP_GENERIC_ID_SMAC_MC,
 	DEVLINK_TRAP_GENERIC_ID_VLAN_TAG_MISMATCH,
@@ -556,6 +559,9 @@ enum devlink_trap_generic_id {
 	DEVLINK_TRAP_GENERIC_ID_MAX = __DEVLINK_TRAP_GENERIC_ID_MAX - 1,
 };
 
+/* All trap groups must be documented in
+ * Documentation/networking/devlink-trap.rst
+ */
 enum devlink_trap_group_generic_id {
 	DEVLINK_TRAP_GROUP_GENERIC_ID_L2_DROPS,
 	DEVLINK_TRAP_GROUP_GENERIC_ID_L3_DROPS,