xref: /linux/Documentation/bpf/map_xskmap.rst (revision a266ef69b890f099069cf51bb40572611c435a54) 
1.. SPDX-License-Identifier: GPL-2.0-only
2.. Copyright (C) 2022 Red Hat, Inc.
3
4===================
5BPF_MAP_TYPE_XSKMAP
6===================
7
8.. note::
9   - ``BPF_MAP_TYPE_XSKMAP`` was introduced in kernel version 4.18
10
11The ``BPF_MAP_TYPE_XSKMAP`` is used as a backend map for XDP BPF helper
12call ``bpf_redirect_map()`` and ``XDP_REDIRECT`` action, like 'devmap' and 'cpumap'.
13This map type redirects raw XDP frames to `AF_XDP`_ sockets (XSKs), a new type of
14address family in the kernel that allows redirection of frames from a driver to
15user space without having to traverse the full network stack. An AF_XDP socket
16binds 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
48Typically an XSKMAP is created per netdev. This map contains an array of XSK File
49Descriptors (FDs). The number of array elements is typically set or adjusted using
50the ``max_entries`` map parameter. For AF_XDP ``max_entries`` is equal to the number
51of queues supported by the netdev.
52
53.. note::
54    Both the map key and map value size must be 4 bytes.
55
56Usage
57=====
58
59Kernel BPF
60----------
61bpf_redirect_map()
62^^^^^^^^^^^^^^^^^^
63.. code-block:: c
64
65    long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
66
67Redirect the packet to the endpoint referenced by ``map`` at index ``key``.
68For ``BPF_MAP_TYPE_XSKMAP`` this map contains references to XSK FDs
69for 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
76bpf_map_lookup_elem()
77^^^^^^^^^^^^^^^^^^^^^
78.. code-block:: c
79
80    void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
81
82XSK entry references of type ``struct xdp_sock *`` can be retrieved using the
83``bpf_map_lookup_elem()`` helper.
84
85User 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
92bpf_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
98XSK entries can be added or updated using the ``bpf_map_update_elem()``
99helper. The ``key`` parameter is equal to the queue_id of the queue the XSK
100is attaching to. And the ``value`` parameter is the FD value of that socket.
101
102Under the hood, the XSKMAP update function uses the XSK FD value to retrieve the
103associated ``struct xdp_sock`` instance.
104
105The 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
111bpf_map_lookup_elem()
112^^^^^^^^^^^^^^^^^^^^^
113.. code-block:: c
114
115    int bpf_map_lookup_elem(int fd, const void *key, void *value)
116
117Returns ``struct xdp_sock *`` or negative error in case of failure.
118
119bpf_map_delete_elem()
120^^^^^^^^^^^^^^^^^^^^^
121.. code-block:: c
122
123    int bpf_map_delete_elem(int fd, const void *key)
124
125XSK entries can be deleted using the ``bpf_map_delete_elem()``
126helper. This helper will return 0 on success, or negative error in case of
127failure.
128
129.. note::
130    When `libxdp`_ deletes an XSK it also removes the associated socket
131    entry from the XSKMAP.
132
133Examples
134========
135Kernel
136------
137
138The 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
161User space
162----------
163
164The 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
179For an example on how create AF_XDP sockets, please see the AF_XDP-example and
180AF_XDP-forwarding programs in the `bpf-examples`_ directory in the `libxdp`_ repository.
181For a detailed explaination 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
193