| .. 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 (e.g., unknown unicast, unregistered multicast) and there are |
| no ports the packets should be flooded to |
| * - ``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. |
