1.. SPDX-License-Identifier: GPL-2.0
2
3============
4Devlink Trap
5============
6
7Background
8==========
9
10Devices capable of offloading the kernel's datapath and perform functions such
11as bridging and routing must also be able to send specific packets to the
12kernel (i.e., the CPU) for processing.
13
14For example, a device acting as a multicast-aware bridge must be able to send
15IGMP membership reports to the kernel for processing by the bridge module.
16Without processing such packets, the bridge module could never populate its
17MDB.
18
19As another example, consider a device acting as router which has received an IP
20packet with a TTL of 1. Upon routing the packet the device must send it to the
21kernel so that it will route it as well and generate an ICMP Time Exceeded
22error datagram. Without letting the kernel route such packets itself, utilities
23such as ``traceroute`` could never work.
24
25The fundamental ability of sending certain packets to the kernel for processing
26is called "packet trapping".
27
28Overview
29========
30
31The ``devlink-trap`` mechanism allows capable device drivers to register their
32supported packet traps with ``devlink`` and report trapped packets to
33``devlink`` for further analysis.
34
35Upon receiving trapped packets, ``devlink`` will perform a per-trap packets and
36bytes accounting and potentially report the packet to user space via a netlink
37event along with all the provided metadata (e.g., trap reason, timestamp, input
38port). This is especially useful for drop traps (see :ref:`Trap-Types`)
39as it allows users to obtain further visibility into packet drops that would
40otherwise be invisible.
41
42The following diagram provides a general overview of ``devlink-trap``::
43
44                                    Netlink event: Packet w/ metadata
45                                                   Or a summary of recent drops
46                                  ^
47                                  |
48         Userspace                |
49        +---------------------------------------------------+
50         Kernel                   |
51                                  |
52                          +-------+--------+
53                          |                |
54                          |  drop_monitor  |
55                          |                |
56                          +-------^--------+
57                                  |
58                                  |
59                                  |
60                             +----+----+
61                             |         |      Kernel's Rx path
62                             | devlink |      (non-drop traps)
63                             |         |
64                             +----^----+      ^
65                                  |           |
66                                  +-----------+
67                                  |
68                          +-------+-------+
69                          |               |
70                          | Device driver |
71                          |               |
72                          +-------^-------+
73         Kernel                   |
74        +---------------------------------------------------+
75         Hardware                 |
76                                  | Trapped packet
77                                  |
78                               +--+---+
79                               |      |
80                               | ASIC |
81                               |      |
82                               +------+
83
84.. _Trap-Types:
85
86Trap Types
87==========
88
89The ``devlink-trap`` mechanism supports the following packet trap types:
90
91  * ``drop``: Trapped packets were dropped by the underlying device. Packets
92    are only processed by ``devlink`` and not injected to the kernel's Rx path.
93    The trap action (see :ref:`Trap-Actions`) can be changed.
94  * ``exception``: Trapped packets were not forwarded as intended by the
95    underlying device due to an exception (e.g., TTL error, missing neighbour
96    entry) and trapped to the control plane for resolution. Packets are
97    processed by ``devlink`` and injected to the kernel's Rx path. Changing the
98    action of such traps is not allowed, as it can easily break the control
99    plane.
100
101.. _Trap-Actions:
102
103Trap Actions
104============
105
106The ``devlink-trap`` mechanism supports the following packet trap actions:
107
108  * ``trap``: The sole copy of the packet is sent to the CPU.
109  * ``drop``: The packet is dropped by the underlying device and a copy is not
110    sent to the CPU.
111
112Generic Packet Traps
113====================
114
115Generic packet traps are used to describe traps that trap well-defined packets
116or packets that are trapped due to well-defined conditions (e.g., TTL error).
117Such traps can be shared by multiple device drivers and their description must
118be added to the following table:
119
120.. list-table:: List of Generic Packet Traps
121   :widths: 5 5 90
122
123   * - Name
124     - Type
125     - Description
126   * - ``source_mac_is_multicast``
127     - ``drop``
128     - Traps incoming packets that the device decided to drop because of a
129       multicast source MAC
130   * - ``vlan_tag_mismatch``
131     - ``drop``
132     - Traps incoming packets that the device decided to drop in case of VLAN
133       tag mismatch: The ingress bridge port is not configured with a PVID and
134       the packet is untagged or prio-tagged
135   * - ``ingress_vlan_filter``
136     - ``drop``
137     - Traps incoming packets that the device decided to drop in case they are
138       tagged with a VLAN that is not configured on the ingress bridge port
139   * - ``ingress_spanning_tree_filter``
140     - ``drop``
141     - Traps incoming packets that the device decided to drop in case the STP
142       state of the ingress bridge port is not "forwarding"
143   * - ``port_list_is_empty``
144     - ``drop``
145     - Traps packets that the device decided to drop in case they need to be
146       flooded (e.g., unknown unicast, unregistered multicast) and there are
147       no ports the packets should be flooded to
148   * - ``port_loopback_filter``
149     - ``drop``
150     - Traps packets that the device decided to drop in case after layer 2
151       forwarding the only port from which they should be transmitted through
152       is the port from which they were received
153   * - ``blackhole_route``
154     - ``drop``
155     - Traps packets that the device decided to drop in case they hit a
156       blackhole route
157   * - ``ttl_value_is_too_small``
158     - ``exception``
159     - Traps unicast packets that should be forwarded by the device whose TTL
160       was decremented to 0 or less
161   * - ``tail_drop``
162     - ``drop``
163     - Traps packets that the device decided to drop because they could not be
164       enqueued to a transmission queue which is full
165   * - ``non_ip``
166     - ``drop``
167     - Traps packets that the device decided to drop because they need to
168       undergo a layer 3 lookup, but are not IP or MPLS packets
169   * - ``uc_dip_over_mc_dmac``
170     - ``drop``
171     - Traps packets that the device decided to drop because they need to be
172       routed and they have a unicast destination IP and a multicast destination
173       MAC
174   * - ``dip_is_loopback_address``
175     - ``drop``
176     - Traps packets that the device decided to drop because they need to be
177       routed and their destination IP is the loopback address (i.e., 127.0.0.0/8
178       and ::1/128)
179   * - ``sip_is_mc``
180     - ``drop``
181     - Traps packets that the device decided to drop because they need to be
182       routed and their source IP is multicast (i.e., 224.0.0.0/8 and ff::/8)
183   * - ``sip_is_loopback_address``
184     - ``drop``
185     - Traps packets that the device decided to drop because they need to be
186       routed and their source IP is the loopback address (i.e., 127.0.0.0/8 and ::1/128)
187   * - ``ip_header_corrupted``
188     - ``drop``
189     - Traps packets that the device decided to drop because they need to be
190       routed and their IP header is corrupted: wrong checksum, wrong IP version
191       or too short Internet Header Length (IHL)
192   * - ``ipv4_sip_is_limited_bc``
193     - ``drop``
194     - Traps packets that the device decided to drop because they need to be
195       routed and their source IP is limited broadcast (i.e., 255.255.255.255/32)
196   * - ``ipv6_mc_dip_reserved_scope``
197     - ``drop``
198     - Traps IPv6 packets that the device decided to drop because they need to
199       be routed and their IPv6 multicast destination IP has a reserved scope
200       (i.e., ffx0::/16)
201   * - ``ipv6_mc_dip_interface_local_scope``
202     - ``drop``
203     - Traps IPv6 packets that the device decided to drop because they need to
204       be routed and their IPv6 multicast destination IP has an interface-local scope
205       (i.e., ffx1::/16)
206   * - ``mtu_value_is_too_small``
207     - ``exception``
208     - Traps packets that should have been routed by the device, but were bigger
209       than the MTU of the egress interface
210   * - ``unresolved_neigh``
211     - ``exception``
212     - Traps packets that did not have a matching IP neighbour after routing
213   * - ``mc_reverse_path_forwarding``
214     - ``exception``
215     - Traps multicast IP packets that failed reverse-path forwarding (RPF)
216       check during multicast routing
217   * - ``reject_route``
218     - ``exception``
219     - Traps packets that hit reject routes (i.e., "unreachable", "prohibit")
220   * - ``ipv4_lpm_miss``
221     - ``exception``
222     - Traps unicast IPv4 packets that did not match any route
223   * - ``ipv6_lpm_miss``
224     - ``exception``
225     - Traps unicast IPv6 packets that did not match any route
226   * - ``non_routable_packet``
227     - ``drop``
228     - Traps packets that the device decided to drop because they are not
229       supposed to be routed. For example, IGMP queries can be flooded by the
230       device in layer 2 and reach the router. Such packets should not be
231       routed and instead dropped
232   * - ``decap_error``
233     - ``exception``
234     - Traps NVE and IPinIP packets that the device decided to drop because of
235       failure during decapsulation (e.g., packet being too short, reserved
236       bits set in VXLAN header)
237   * - ``overlay_smac_is_mc``
238     - ``drop``
239     - Traps NVE packets that the device decided to drop because their overlay
240       source MAC is multicast
241   * - ``ingress_flow_action_drop``
242     - ``drop``
243     - Traps packets dropped during processing of ingress flow action drop
244   * - ``egress_flow_action_drop``
245     - ``drop``
246     - Traps packets dropped during processing of egress flow action drop
247
248Driver-specific Packet Traps
249============================
250
251Device drivers can register driver-specific packet traps, but these must be
252clearly documented. Such traps can correspond to device-specific exceptions and
253help debug packet drops caused by these exceptions. The following list includes
254links to the description of driver-specific traps registered by various device
255drivers:
256
257  * :doc:`netdevsim`
258  * :doc:`mlxsw`
259
260.. _Generic-Packet-Trap-Groups:
261
262Generic Packet Trap Groups
263==========================
264
265Generic packet trap groups are used to aggregate logically related packet
266traps. These groups allow the user to batch operations such as setting the trap
267action of all member traps. In addition, ``devlink-trap`` can report aggregated
268per-group packets and bytes statistics, in case per-trap statistics are too
269narrow. The description of these groups must be added to the following table:
270
271.. list-table:: List of Generic Packet Trap Groups
272   :widths: 10 90
273
274   * - Name
275     - Description
276   * - ``l2_drops``
277     - Contains packet traps for packets that were dropped by the device during
278       layer 2 forwarding (i.e., bridge)
279   * - ``l3_drops``
280     - Contains packet traps for packets that were dropped by the device or hit
281       an exception (e.g., TTL error) during layer 3 forwarding
282   * - ``buffer_drops``
283     - Contains packet traps for packets that were dropped by the device due to
284       an enqueue decision
285   * - ``tunnel_drops``
286     - Contains packet traps for packets that were dropped by the device during
287       tunnel encapsulation / decapsulation
288   * - ``acl_drops``
289     - Contains packet traps for packets that were dropped by the device during
290       ACL processing
291
292Packet Trap Policers
293====================
294
295As previously explained, the underlying device can trap certain packets to the
296CPU for processing. In most cases, the underlying device is capable of handling
297packet rates that are several orders of magnitude higher compared to those that
298can be handled by the CPU.
299
300Therefore, in order to prevent the underlying device from overwhelming the CPU,
301devices usually include packet trap policers that are able to police the
302trapped packets to rates that can be handled by the CPU.
303
304The ``devlink-trap`` mechanism allows capable device drivers to register their
305supported packet trap policers with ``devlink``. The device driver can choose
306to associate these policers with supported packet trap groups (see
307:ref:`Generic-Packet-Trap-Groups`) during its initialization, thereby exposing
308its default control plane policy to user space.
309
310Device drivers should allow user space to change the parameters of the policers
311(e.g., rate, burst size) as well as the association between the policers and
312trap groups by implementing the relevant callbacks.
313
314If possible, device drivers should implement a callback that allows user space
315to retrieve the number of packets that were dropped by the policer because its
316configured policy was violated.
317
318Testing
319=======
320
321See ``tools/testing/selftests/drivers/net/netdevsim/devlink_trap.sh`` for a
322test covering the core infrastructure. Test cases should be added for any new
323functionality.
324
325Device drivers should focus their tests on device-specific functionality, such
326as the triggering of supported packet traps.
327