1 .. SPDX-License-Identifier: GPL-2.0-only
2 .. Copyright (C) 2022 Red Hat, Inc.
3
4 ===================
5 BPF_MAP_TYPE_XSKMAP
6 ===================
7
8 .. note::
9 - ``BPF_MAP_TYPE_XSKMAP`` was introduced in kernel version 4.18
10
11 The ``BPF_MAP_TYPE_XSKMAP`` is used as a backend map for XDP BPF helper
12 call ``bpf_redirect_map()`` and ``XDP_REDIRECT`` action, like 'devmap' and 'cpumap'.
13 This map type redirects raw XDP frames to `AF_XDP`_ sockets (XSKs), a new type of
14 address family in the kernel that allows redirection of frames from a driver to
15 user space without having to traverse the full network stack. An AF_XDP socket
16 binds to a single netdev queue. A mapping of XSKs to queues is shown below:
17
18 .. code-block:: none
19
20 +---------------------------------------------------+
21 | xsk A | xsk B | xsk C |<---+ User space
22 =========================================================|==========
23 | Queue 0 | Queue 1 | Queue 2 | | Kernel
24 +---------------------------------------------------+ |
25 | Netdev eth0 | |
26 +---------------------------------------------------+ |
27 | +=============+ | |
28 | | key | xsk | | |
29 | +---------+ +=============+ | |
30 | | | | 0 | xsk A | | |
31 | | | +-------------+ | |
32 | | | | 1 | xsk B | | |
33 | | BPF |-- redirect -->+-------------+-------------+
34 | | prog | | 2 | xsk C | |
35 | | | +-------------+ |
36 | | | |
37 | | | |
38 | +---------+ |
39 | |
40 +---------------------------------------------------+
41
42 .. note::
43 An AF_XDP socket that is bound to a certain <netdev/queue_id> will *only*
44 accept XDP frames from that <netdev/queue_id>. If an XDP program tries to redirect
45 from a <netdev/queue_id> other than what the socket is bound to, the frame will
46 not be received on the socket.
47
48 Typically an XSKMAP is created per netdev. This map contains an array of XSK File
49 Descriptors (FDs). The number of array elements is typically set or adjusted using
50 the ``max_entries`` map parameter. For AF_XDP ``max_entries`` is equal to the number
51 of queues supported by the netdev.
52
53 .. note::
54 Both the map key and map value size must be 4 bytes.
55
56 Usage
57 =====
58
59 Kernel BPF
60 ----------
61 bpf_redirect_map()
62 ^^^^^^^^^^^^^^^^^^
63 .. code-block:: c
64
65 long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
66
67 Redirect the packet to the endpoint referenced by ``map`` at index ``key``.
68 For ``BPF_MAP_TYPE_XSKMAP`` this map contains references to XSK FDs
69 for sockets attached to a netdev's queues.
70
71 .. note::
72 If the map is empty at an index, the packet is dropped. This means that it is
73 necessary to have an XDP program loaded with at least one XSK in the
74 XSKMAP to be able to get any traffic to user space through the socket.
75
76 bpf_map_lookup_elem()
77 ^^^^^^^^^^^^^^^^^^^^^
78 .. code-block:: c
79
80 void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
81
82 XSK entry references of type ``struct xdp_sock *`` can be retrieved using the
83 ``bpf_map_lookup_elem()`` helper.
84
85 User space
86 ----------
87 .. note::
88 XSK entries can only be updated/deleted from user space and not from
89 a BPF program. Trying to call these functions from a kernel BPF program will
90 result in the program failing to load and a verifier warning.
91
92 bpf_map_update_elem()
93 ^^^^^^^^^^^^^^^^^^^^^
94 .. code-block:: c
95
96 int bpf_map_update_elem(int fd, const void *key, const void *value, __u64 flags)
97
98 XSK entries can be added or updated using the ``bpf_map_update_elem()``
99 helper. The ``key`` parameter is equal to the queue_id of the queue the XSK
100 is attaching to. And the ``value`` parameter is the FD value of that socket.
101
102 Under the hood, the XSKMAP update function uses the XSK FD value to retrieve the
103 associated ``struct xdp_sock`` instance.
104
105 The flags argument can be one of the following:
106
107 - BPF_ANY: Create a new element or update an existing element.
108 - BPF_NOEXIST: Create a new element only if it did not exist.
109 - BPF_EXIST: Update an existing element.
110
111 bpf_map_lookup_elem()
112 ^^^^^^^^^^^^^^^^^^^^^
113 .. code-block:: c
114
115 int bpf_map_lookup_elem(int fd, const void *key, void *value)
116
117 Returns ``struct xdp_sock *`` or negative error in case of failure.
118
119 bpf_map_delete_elem()
120 ^^^^^^^^^^^^^^^^^^^^^
121 .. code-block:: c
122
123 int bpf_map_delete_elem(int fd, const void *key)
124
125 XSK entries can be deleted using the ``bpf_map_delete_elem()``
126 helper. This helper will return 0 on success, or negative error in case of
127 failure.
128
129 .. note::
130 When `libxdp`_ deletes an XSK it also removes the associated socket
131 entry from the XSKMAP.
132
133 Examples
134 ========
135 Kernel
136 ------
137
138 The following code snippet shows how to declare a ``BPF_MAP_TYPE_XSKMAP`` called
139 ``xsks_map`` and how to redirect packets to an XSK.
140
141 .. code-block:: c
142
143 struct {
144 __uint(type, BPF_MAP_TYPE_XSKMAP);
145 __type(key, __u32);
146 __type(value, __u32);
147 __uint(max_entries, 64);
148 } xsks_map SEC(".maps");
149
150
151 SEC("xdp")
152 int xsk_redir_prog(struct xdp_md *ctx)
153 {
154 __u32 index = ctx->rx_queue_index;
155
156 if (bpf_map_lookup_elem(&xsks_map, &index))
157 return bpf_redirect_map(&xsks_map, index, 0);
158 return XDP_PASS;
159 }
160
161 User space
162 ----------
163
164 The following code snippet shows how to update an XSKMAP with an XSK entry.
165
166 .. code-block:: c
167
168 int update_xsks_map(struct bpf_map *xsks_map, int queue_id, int xsk_fd)
169 {
170 int ret;
171
172 ret = bpf_map_update_elem(bpf_map__fd(xsks_map), &queue_id, &xsk_fd, 0);
173 if (ret < 0)
174 fprintf(stderr, "Failed to update xsks_map: %s\n", strerror(errno));
175
176 return ret;
177 }
178
179 For an example on how create AF_XDP sockets, please see the AF_XDP-example and
180 AF_XDP-forwarding programs in the `bpf-examples`_ directory in the `libxdp`_ repository.
181 For a detailed explanation of the AF_XDP interface please see:
182
183 - `libxdp-readme`_.
184 - `AF_XDP`_ kernel documentation.
185
186 .. note::
187 The most comprehensive resource for using XSKMAPs and AF_XDP is `libxdp`_.
188
189 .. _libxdp: https://github.com/xdp-project/xdp-tools/tree/master/lib/libxdp
190 .. _AF_XDP: https://www.kernel.org/doc/html/latest/networking/af_xdp.html
191 .. _bpf-examples: https://github.com/xdp-project/bpf-examples
192 .. _libxdp-readme: https://github.com/xdp-project/xdp-tools/tree/master/lib/libxdp#using-af_xdp-sockets
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