1 .. SPDX-License-Identifier: GPL-2.0
7 This document describes how an eXpress Data Path (XDP) program can access
8 hardware metadata related to a packet using a set of helper functions,
9 and how it can pass that metadata on to other consumers.
14 XDP has access to a set of kfuncs to manipulate the metadata in an XDP frame.
15 Every device driver that wishes to expose additional packet metadata can
16 implement these kfuncs. The set of kfuncs is declared in ``include/net/xdp.h``
17 via ``XDP_METADATA_KFUNC_xxx``.
19 Currently, the following kfuncs are supported. In the future, as more
20 metadata is supported, this set will grow:
22 .. kernel-doc:: net/core/xdp.c
23 :identifiers: bpf_xdp_metadata_rx_timestamp
25 .. kernel-doc:: net/core/xdp.c
26 :identifiers: bpf_xdp_metadata_rx_hash
28 .. kernel-doc:: net/core/xdp.c
29 :identifiers: bpf_xdp_metadata_rx_vlan_tag
31 An XDP program can use these kfuncs to read the metadata into stack
32 variables for its own consumption. Or, to pass the metadata on to other
33 consumers, an XDP program can store it into the metadata area carried
34 ahead of the packet. Not all packets will necessary have the requested
35 metadata available in which case the driver returns ``-ENODATA``.
37 Not all kfuncs have to be implemented by the device driver; when not
38 implemented, the default ones that return ``-EOPNOTSUPP`` will be used
39 to indicate the device driver have not implemented this kfunc.
42 Within an XDP frame, the metadata layout (accessed via ``xdp_buff``) is
45 +----------+-----------------+------+
46 | headroom | custom metadata | data |
47 +----------+-----------------+------+
50 xdp_buff->data_meta xdp_buff->data
52 An XDP program can store individual metadata items into this ``data_meta``
53 area in whichever format it chooses. Later consumers of the metadata
54 will have to agree on the format by some out of band contract (like for
55 the AF_XDP use case, see below).
60 :doc:`af_xdp` use-case implies that there is a contract between the BPF
61 program that redirects XDP frames into the ``AF_XDP`` socket (``XSK``) and
62 the final consumer. Thus the BPF program manually allocates a fixed number of
63 bytes out of metadata via ``bpf_xdp_adjust_meta`` and calls a subset
64 of kfuncs to populate it. The userspace ``XSK`` consumer computes
65 ``xsk_umem__get_data() - METADATA_SIZE`` to locate that metadata.
66 Note, ``xsk_umem__get_data`` is defined in ``libxdp`` and
67 ``METADATA_SIZE`` is an application-specific constant (``AF_XDP`` receive
68 descriptor does _not_ explicitly carry the size of the metadata).
70 Here is the ``AF_XDP`` consumer layout (note missing ``data_meta`` pointer)::
72 +----------+-----------------+------+
73 | headroom | custom metadata | data |
74 +----------+-----------------+------+
82 This is the path where the packets processed by the XDP program are passed
83 into the kernel. The kernel creates the ``skb`` out of the ``xdp_buff``
84 contents. Currently, every driver has custom kernel code to parse
85 the descriptors and populate ``skb`` metadata when doing this ``xdp_buff->skb``
86 conversion, and the XDP metadata is not used by the kernel when building
87 ``skbs``. However, TC-BPF programs can access the XDP metadata area using
88 the ``data_meta`` pointer.
90 In the future, we'd like to support a case where an XDP program
91 can override some of the metadata used for building ``skbs``.
96 ``bpf_redirect_map`` can redirect the frame to a different device.
97 Some devices (like virtual ethernet links) support running a second XDP
98 program after the redirect. However, the final consumer doesn't have
99 access to the original hardware descriptor and can't access any of
100 the original metadata. The same applies to XDP programs installed
101 into devmaps and cpumaps.
103 This means that for redirected packets only custom metadata is
104 currently supported, which has to be prepared by the initial XDP program
105 before redirect. If the frame is eventually passed to the kernel, the
106 ``skb`` created from such a frame won't have any hardware metadata populated
107 in its ``skb``. If such a packet is later redirected into an ``XSK``,
108 that will also only have access to the custom metadata.
113 Adding programs that access metadata kfuncs to the ``BPF_MAP_TYPE_PROG_ARRAY``
114 is currently not supported.
119 It is possible to query which kfunc the particular netdev implements via
120 netlink. See ``xdp-rx-metadata-features`` attribute set in
121 ``Documentation/netlink/specs/netdev.yaml``.
126 See ``tools/testing/selftests/bpf/progs/xdp_metadata.c`` and
127 ``tools/testing/selftests/bpf/prog_tests/xdp_metadata.c`` for an example of
128 BPF program that handles XDP metadata.