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ReStructuredText
327 lines
13 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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============
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Devlink Trap
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============
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Background
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==========
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Devices capable of offloading the kernel's datapath and perform functions such
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as bridging and routing must also be able to send specific packets to the
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kernel (i.e., the CPU) for processing.
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For example, a device acting as a multicast-aware bridge must be able to send
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IGMP membership reports to the kernel for processing by the bridge module.
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Without processing such packets, the bridge module could never populate its
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MDB.
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As another example, consider a device acting as router which has received an IP
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packet with a TTL of 1. Upon routing the packet the device must send it to the
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kernel so that it will route it as well and generate an ICMP Time Exceeded
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error datagram. Without letting the kernel route such packets itself, utilities
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such as ``traceroute`` could never work.
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The fundamental ability of sending certain packets to the kernel for processing
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is called "packet trapping".
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Overview
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========
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The ``devlink-trap`` mechanism allows capable device drivers to register their
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supported packet traps with ``devlink`` and report trapped packets to
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``devlink`` for further analysis.
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Upon receiving trapped packets, ``devlink`` will perform a per-trap packets and
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bytes accounting and potentially report the packet to user space via a netlink
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event along with all the provided metadata (e.g., trap reason, timestamp, input
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port). This is especially useful for drop traps (see :ref:`Trap-Types`)
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as it allows users to obtain further visibility into packet drops that would
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otherwise be invisible.
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The following diagram provides a general overview of ``devlink-trap``::
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Netlink event: Packet w/ metadata
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Or a summary of recent drops
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^
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Userspace |
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+---------------------------------------------------+
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Kernel |
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+-------+--------+
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| drop_monitor |
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+-------^--------+
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+----+----+
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| | Kernel's Rx path
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| devlink | (non-drop traps)
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+----^----+ ^
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+-----------+
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+-------+-------+
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| Device driver |
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+-------^-------+
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Kernel |
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+---------------------------------------------------+
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Hardware |
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| Trapped packet
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+--+---+
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| ASIC |
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+------+
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.. _Trap-Types:
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Trap Types
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==========
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The ``devlink-trap`` mechanism supports the following packet trap types:
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* ``drop``: Trapped packets were dropped by the underlying device. Packets
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are only processed by ``devlink`` and not injected to the kernel's Rx path.
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The trap action (see :ref:`Trap-Actions`) can be changed.
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* ``exception``: Trapped packets were not forwarded as intended by the
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underlying device due to an exception (e.g., TTL error, missing neighbour
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entry) and trapped to the control plane for resolution. Packets are
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processed by ``devlink`` and injected to the kernel's Rx path. Changing the
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action of such traps is not allowed, as it can easily break the control
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plane.
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.. _Trap-Actions:
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Trap Actions
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============
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The ``devlink-trap`` mechanism supports the following packet trap actions:
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* ``trap``: The sole copy of the packet is sent to the CPU.
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* ``drop``: The packet is dropped by the underlying device and a copy is not
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sent to the CPU.
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Generic Packet Traps
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====================
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Generic packet traps are used to describe traps that trap well-defined packets
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or packets that are trapped due to well-defined conditions (e.g., TTL error).
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Such traps can be shared by multiple device drivers and their description must
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be added to the following table:
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.. list-table:: List of Generic Packet Traps
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:widths: 5 5 90
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* - Name
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- Type
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- Description
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* - ``source_mac_is_multicast``
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- ``drop``
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- Traps incoming packets that the device decided to drop because of a
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multicast source MAC
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* - ``vlan_tag_mismatch``
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- ``drop``
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- Traps incoming packets that the device decided to drop in case of VLAN
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tag mismatch: The ingress bridge port is not configured with a PVID and
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the packet is untagged or prio-tagged
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* - ``ingress_vlan_filter``
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- ``drop``
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- Traps incoming packets that the device decided to drop in case they are
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tagged with a VLAN that is not configured on the ingress bridge port
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* - ``ingress_spanning_tree_filter``
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- ``drop``
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- Traps incoming packets that the device decided to drop in case the STP
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state of the ingress bridge port is not "forwarding"
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* - ``port_list_is_empty``
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- ``drop``
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- Traps packets that the device decided to drop in case they need to be
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flooded (e.g., unknown unicast, unregistered multicast) and there are
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no ports the packets should be flooded to
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* - ``port_loopback_filter``
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- ``drop``
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- Traps packets that the device decided to drop in case after layer 2
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forwarding the only port from which they should be transmitted through
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is the port from which they were received
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* - ``blackhole_route``
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- ``drop``
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- Traps packets that the device decided to drop in case they hit a
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blackhole route
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* - ``ttl_value_is_too_small``
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- ``exception``
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- Traps unicast packets that should be forwarded by the device whose TTL
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was decremented to 0 or less
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* - ``tail_drop``
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- ``drop``
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- Traps packets that the device decided to drop because they could not be
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enqueued to a transmission queue which is full
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* - ``non_ip``
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- ``drop``
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- Traps packets that the device decided to drop because they need to
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undergo a layer 3 lookup, but are not IP or MPLS packets
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* - ``uc_dip_over_mc_dmac``
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- ``drop``
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- Traps packets that the device decided to drop because they need to be
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routed and they have a unicast destination IP and a multicast destination
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MAC
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* - ``dip_is_loopback_address``
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- ``drop``
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- Traps packets that the device decided to drop because they need to be
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routed and their destination IP is the loopback address (i.e., 127.0.0.0/8
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and ::1/128)
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* - ``sip_is_mc``
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- ``drop``
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- Traps packets that the device decided to drop because they need to be
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routed and their source IP is multicast (i.e., 224.0.0.0/8 and ff::/8)
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* - ``sip_is_loopback_address``
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- ``drop``
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- Traps packets that the device decided to drop because they need to be
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routed and their source IP is the loopback address (i.e., 127.0.0.0/8 and ::1/128)
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* - ``ip_header_corrupted``
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- ``drop``
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- Traps packets that the device decided to drop because they need to be
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routed and their IP header is corrupted: wrong checksum, wrong IP version
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or too short Internet Header Length (IHL)
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* - ``ipv4_sip_is_limited_bc``
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- ``drop``
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- Traps packets that the device decided to drop because they need to be
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routed and their source IP is limited broadcast (i.e., 255.255.255.255/32)
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* - ``ipv6_mc_dip_reserved_scope``
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- ``drop``
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- Traps IPv6 packets that the device decided to drop because they need to
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be routed and their IPv6 multicast destination IP has a reserved scope
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(i.e., ffx0::/16)
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* - ``ipv6_mc_dip_interface_local_scope``
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- ``drop``
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- Traps IPv6 packets that the device decided to drop because they need to
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be routed and their IPv6 multicast destination IP has an interface-local scope
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(i.e., ffx1::/16)
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* - ``mtu_value_is_too_small``
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- ``exception``
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- Traps packets that should have been routed by the device, but were bigger
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than the MTU of the egress interface
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* - ``unresolved_neigh``
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- ``exception``
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- Traps packets that did not have a matching IP neighbour after routing
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* - ``mc_reverse_path_forwarding``
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- ``exception``
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- Traps multicast IP packets that failed reverse-path forwarding (RPF)
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check during multicast routing
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* - ``reject_route``
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- ``exception``
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- Traps packets that hit reject routes (i.e., "unreachable", "prohibit")
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* - ``ipv4_lpm_miss``
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- ``exception``
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- Traps unicast IPv4 packets that did not match any route
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* - ``ipv6_lpm_miss``
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- ``exception``
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- Traps unicast IPv6 packets that did not match any route
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* - ``non_routable_packet``
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- ``drop``
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- Traps packets that the device decided to drop because they are not
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supposed to be routed. For example, IGMP queries can be flooded by the
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device in layer 2 and reach the router. Such packets should not be
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routed and instead dropped
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* - ``decap_error``
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- ``exception``
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- Traps NVE and IPinIP packets that the device decided to drop because of
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failure during decapsulation (e.g., packet being too short, reserved
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bits set in VXLAN header)
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* - ``overlay_smac_is_mc``
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- ``drop``
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- Traps NVE packets that the device decided to drop because their overlay
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source MAC is multicast
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* - ``ingress_flow_action_drop``
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- ``drop``
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- Traps packets dropped during processing of ingress flow action drop
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* - ``egress_flow_action_drop``
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- ``drop``
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- Traps packets dropped during processing of egress flow action drop
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Driver-specific Packet Traps
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============================
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Device drivers can register driver-specific packet traps, but these must be
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clearly documented. Such traps can correspond to device-specific exceptions and
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help debug packet drops caused by these exceptions. The following list includes
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links to the description of driver-specific traps registered by various device
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drivers:
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* :doc:`netdevsim`
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* :doc:`mlxsw`
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.. _Generic-Packet-Trap-Groups:
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Generic Packet Trap Groups
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==========================
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Generic packet trap groups are used to aggregate logically related packet
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traps. These groups allow the user to batch operations such as setting the trap
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action of all member traps. In addition, ``devlink-trap`` can report aggregated
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per-group packets and bytes statistics, in case per-trap statistics are too
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narrow. The description of these groups must be added to the following table:
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.. list-table:: List of Generic Packet Trap Groups
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:widths: 10 90
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* - Name
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- Description
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* - ``l2_drops``
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- Contains packet traps for packets that were dropped by the device during
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layer 2 forwarding (i.e., bridge)
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* - ``l3_drops``
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- Contains packet traps for packets that were dropped by the device or hit
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an exception (e.g., TTL error) during layer 3 forwarding
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* - ``buffer_drops``
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- Contains packet traps for packets that were dropped by the device due to
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an enqueue decision
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* - ``tunnel_drops``
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- Contains packet traps for packets that were dropped by the device during
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tunnel encapsulation / decapsulation
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* - ``acl_drops``
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- Contains packet traps for packets that were dropped by the device during
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ACL processing
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Packet Trap Policers
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====================
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As previously explained, the underlying device can trap certain packets to the
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CPU for processing. In most cases, the underlying device is capable of handling
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packet rates that are several orders of magnitude higher compared to those that
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can be handled by the CPU.
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Therefore, in order to prevent the underlying device from overwhelming the CPU,
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devices usually include packet trap policers that are able to police the
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trapped packets to rates that can be handled by the CPU.
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The ``devlink-trap`` mechanism allows capable device drivers to register their
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supported packet trap policers with ``devlink``. The device driver can choose
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to associate these policers with supported packet trap groups (see
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:ref:`Generic-Packet-Trap-Groups`) during its initialization, thereby exposing
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its default control plane policy to user space.
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Device drivers should allow user space to change the parameters of the policers
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(e.g., rate, burst size) as well as the association between the policers and
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trap groups by implementing the relevant callbacks.
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If possible, device drivers should implement a callback that allows user space
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to retrieve the number of packets that were dropped by the policer because its
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configured policy was violated.
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Testing
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=======
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See ``tools/testing/selftests/drivers/net/netdevsim/devlink_trap.sh`` for a
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test covering the core infrastructure. Test cases should be added for any new
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functionality.
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Device drivers should focus their tests on device-specific functionality, such
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as the triggering of supported packet traps.
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