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diff --git a/Documentation/networking/devlink-trap.rst b/Documentation/networking/devlink-trap.rst new file mode 100644 index 000000000000..c20c7c483664 --- /dev/null +++ b/Documentation/networking/devlink-trap.rst @@ -0,0 +1,208 @@ +.. 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. |