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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | ============ | |
4 | Devlink Trap | |
5 | ============ | |
6 | ||
7 | Background | |
8 | ========== | |
9 | ||
10 | Devices capable of offloading the kernel's datapath and perform functions such | |
11 | as bridging and routing must also be able to send specific packets to the | |
12 | kernel (i.e., the CPU) for processing. | |
13 | ||
14 | For example, a device acting as a multicast-aware bridge must be able to send | |
15 | IGMP membership reports to the kernel for processing by the bridge module. | |
16 | Without processing such packets, the bridge module could never populate its | |
17 | MDB. | |
18 | ||
19 | As another example, consider a device acting as router which has received an IP | |
20 | packet with a TTL of 1. Upon routing the packet the device must send it to the | |
21 | kernel so that it will route it as well and generate an ICMP Time Exceeded | |
22 | error datagram. Without letting the kernel route such packets itself, utilities | |
23 | such as ``traceroute`` could never work. | |
24 | ||
25 | The fundamental ability of sending certain packets to the kernel for processing | |
26 | is called "packet trapping". | |
27 | ||
28 | Overview | |
29 | ======== | |
30 | ||
31 | The ``devlink-trap`` mechanism allows capable device drivers to register their | |
32 | supported packet traps with ``devlink`` and report trapped packets to | |
33 | ``devlink`` for further analysis. | |
34 | ||
35 | Upon receiving trapped packets, ``devlink`` will perform a per-trap packets and | |
36 | bytes accounting and potentially report the packet to user space via a netlink | |
37 | event along with all the provided metadata (e.g., trap reason, timestamp, input | |
38 | port). This is especially useful for drop traps (see :ref:`Trap-Types`) | |
39 | as it allows users to obtain further visibility into packet drops that would | |
40 | otherwise be invisible. | |
41 | ||
42 | The 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 | ||
86 | Trap Types | |
87 | ========== | |
88 | ||
89 | The ``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 | ||
103 | Trap Actions | |
104 | ============ | |
105 | ||
106 | The ``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 | ||
112 | Generic Packet Traps | |
113 | ==================== | |
114 | ||
115 | Generic packet traps are used to describe traps that trap well-defined packets | |
116 | or packets that are trapped due to well-defined conditions (e.g., TTL error). | |
117 | Such traps can be shared by multiple device drivers and their description must | |
118 | be 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 | |
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146 | flooded (e.g., unknown unicast, unregistered multicast) and there are |
147 | no ports the packets should be flooded to | |
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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 | ||
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166 | Driver-specific Packet Traps |
167 | ============================ | |
168 | ||
169 | Device drivers can register driver-specific packet traps, but these must be | |
170 | clearly documented. Such traps can correspond to device-specific exceptions and | |
171 | help debug packet drops caused by these exceptions. The following list includes | |
172 | links to the description of driver-specific traps registered by various device | |
173 | drivers: | |
174 | ||
175 | * :doc:`/devlink-trap-netdevsim` | |
176 | ||
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177 | Generic Packet Trap Groups |
178 | ========================== | |
179 | ||
180 | Generic packet trap groups are used to aggregate logically related packet | |
181 | traps. These groups allow the user to batch operations such as setting the trap | |
182 | action of all member traps. In addition, ``devlink-trap`` can report aggregated | |
183 | per-group packets and bytes statistics, in case per-trap statistics are too | |
184 | narrow. The description of these groups must be added to the following table: | |
185 | ||
186 | .. list-table:: List of Generic Packet Trap Groups | |
187 | :widths: 10 90 | |
188 | ||
189 | * - Name | |
190 | - Description | |
191 | * - ``l2_drops`` | |
192 | - Contains packet traps for packets that were dropped by the device during | |
193 | layer 2 forwarding (i.e., bridge) | |
194 | * - ``l3_drops`` | |
195 | - Contains packet traps for packets that were dropped by the device or hit | |
196 | an exception (e.g., TTL error) during layer 3 forwarding | |
197 | * - ``buffer_drops`` | |
198 | - Contains packet traps for packets that were dropped by the device due to | |
199 | an enqueue decision | |
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200 | |
201 | Testing | |
202 | ======= | |
203 | ||
204 | See ``tools/testing/selftests/drivers/net/netdevsim/devlink_trap.sh`` for a | |
205 | test covering the core infrastructure. Test cases should be added for any new | |
206 | functionality. | |
207 | ||
208 | Device drivers should focus their tests on device-specific functionality, such | |
209 | as the triggering of supported packet traps. |