--- /dev/null
+.. 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
+
+Driver-specific Packet Traps
+============================
+
+Device drivers can register driver-specific packet traps, but these must be
+clearly documented. Such traps can correspond to device-specific exceptions and
+help debug packet drops caused by these exceptions. The following list includes
+links to the description of driver-specific traps registered by various device
+drivers:
+
+ * :doc:`/devlink-trap-netdevsim`
+
+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
+
+Testing
+=======
+
+See ``tools/testing/selftests/drivers/net/netdevsim/devlink_trap.sh`` for a
+test covering the core infrastructure. Test cases should be added for any new
+functionality.
+
+Device drivers should focus their tests on device-specific functionality, such
+as the triggering of supported packet traps.
git.samba.org / sfrench / cifs-2.6.git / blobdiff