docs/bpf: Add BPF_MAP_TYPE_XSKMAP documentation

Add documentation for BPF_MAP_TYPE_XSKMAP including kernel version introduced,
usage and examples.
Signed-off-by: Maryam Tahhan <mtahhan@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221123090043.83945-1-mtahhan@redhat.com

Documentation/bpf/map_xskmap.rst

+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Red Hat, Inc.
+
+===================
+BPF_MAP_TYPE_XSKMAP
+===================
+
+.. note::
+   - ``BPF_MAP_TYPE_XSKMAP`` was introduced in kernel version 4.18
+
+The ``BPF_MAP_TYPE_XSKMAP`` is used as a backend map for XDP BPF helper
+call ``bpf_redirect_map()`` and ``XDP_REDIRECT`` action, like 'devmap' and 'cpumap'.
+This map type redirects raw XDP frames to `AF_XDP`_ sockets (XSKs), a new type of
+address family in the kernel that allows redirection of frames from a driver to
+user space without having to traverse the full network stack. An AF_XDP socket
+binds to a single netdev queue. A mapping of XSKs to queues is shown below:
+
+.. code-block:: none
+
+    +---------------------------------------------------+
+    |     xsk A      |     xsk B       |      xsk C     |<---+ User space
+    =========================================================|==========
+    |    Queue 0     |     Queue 1     |     Queue 2    |    |  Kernel
+    +---------------------------------------------------+    |
+    |                  Netdev eth0                      |    |
+    +---------------------------------------------------+    |
+    |                            +=============+        |    |
+    |                            | key |  xsk  |        |    |
+    |  +---------+               +=============+        |    |
+    |  |         |               |  0  | xsk A |        |    |
+    |  |         |               +-------------+        |    |
+    |  |         |               |  1  | xsk B |        |    |
+    |  | BPF     |-- redirect -->+-------------+-------------+
+    |  | prog    |               |  2  | xsk C |        |
+    |  |         |               +-------------+        |
+    |  |         |                                      |
+    |  |         |                                      |
+    |  +---------+                                      |
+    |                                                   |
+    +---------------------------------------------------+
+
+.. note::
+    An AF_XDP socket that is bound to a certain <netdev/queue_id> will *only*
+    accept XDP frames from that <netdev/queue_id>. If an XDP program tries to redirect
+    from a <netdev/queue_id> other than what the socket is bound to, the frame will
+    not be received on the socket.
+
+Typically an XSKMAP is created per netdev. This map contains an array of XSK File
+Descriptors (FDs). The number of array elements is typically set or adjusted using
+the ``max_entries`` map parameter. For AF_XDP ``max_entries`` is equal to the number
+of queues supported by the netdev.
+
+.. note::
+    Both the map key and map value size must be 4 bytes.
+
+Usage
+=====
+
+Kernel BPF
+----------
+bpf_redirect_map()
+^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+    long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
+
+Redirect the packet to the endpoint referenced by ``map`` at index ``key``.
+For ``BPF_MAP_TYPE_XSKMAP`` this map contains references to XSK FDs
+for sockets attached to a netdev's queues.
+
+.. note::
+    If the map is empty at an index, the packet is dropped. This means that it is
+    necessary to have an XDP program loaded with at least one XSK in the
+    XSKMAP to be able to get any traffic to user space through the socket.
+
+bpf_map_lookup_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+    void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+XSK entry references of type ``struct xdp_sock *`` can be retrieved using the
+``bpf_map_lookup_elem()`` helper.
+
+User space
+----------
+.. note::
+    XSK entries can only be updated/deleted from user space and not from
+    a BPF program. Trying to call these functions from a kernel BPF program will
+    result in the program failing to load and a verifier warning.
+
+bpf_map_update_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+	int bpf_map_update_elem(int fd, const void *key, const void *value, __u64 flags)
+
+XSK entries can be added or updated using the ``bpf_map_update_elem()``
+helper. The ``key`` parameter is equal to the queue_id of the queue the XSK
+is attaching to. And the ``value`` parameter is the FD value of that socket.
+
+Under the hood, the XSKMAP update function uses the XSK FD value to retrieve the
+associated ``struct xdp_sock`` instance.
+
+The flags argument can be one of the following:
+
+- BPF_ANY: Create a new element or update an existing element.
+- BPF_NOEXIST: Create a new element only if it did not exist.
+- BPF_EXIST: Update an existing element.
+
+bpf_map_lookup_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+    int bpf_map_lookup_elem(int fd, const void *key, void *value)
+
+Returns ``struct xdp_sock *`` or negative error in case of failure.
+
+bpf_map_delete_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+    int bpf_map_delete_elem(int fd, const void *key)
+
+XSK entries can be deleted using the ``bpf_map_delete_elem()``
+helper. This helper will return 0 on success, or negative error in case of
+failure.
+
+.. note::
+    When `libxdp`_ deletes an XSK it also removes the associated socket
+    entry from the XSKMAP.
+
+Examples
+========
+Kernel
+------
+
+The following code snippet shows how to declare a ``BPF_MAP_TYPE_XSKMAP`` called
+``xsks_map`` and how to redirect packets to an XSK.
+
+.. code-block:: c
+
+	struct {
+		__uint(type, BPF_MAP_TYPE_XSKMAP);
+		__type(key, __u32);
+		__type(value, __u32);
+		__uint(max_entries, 64);
+	} xsks_map SEC(".maps");
+
+
+	SEC("xdp")
+	int xsk_redir_prog(struct xdp_md *ctx)
+	{
+		__u32 index = ctx->rx_queue_index;
+
+		if (bpf_map_lookup_elem(&xsks_map, &index))
+			return bpf_redirect_map(&xsks_map, index, 0);
+		return XDP_PASS;
+	}
+
+User space
+----------
+
+The following code snippet shows how to update an XSKMAP with an XSK entry.
+
+.. code-block:: c
+
+	int update_xsks_map(struct bpf_map *xsks_map, int queue_id, int xsk_fd)
+	{
+		int ret;
+
+		ret = bpf_map_update_elem(bpf_map__fd(xsks_map), &queue_id, &xsk_fd, 0);
+		if (ret < 0)
+			fprintf(stderr, "Failed to update xsks_map: %s\n", strerror(errno));
+
+		return ret;
+	}
+
+For an example on how create AF_XDP sockets, please see the AF_XDP-example and
+AF_XDP-forwarding programs in the `bpf-examples`_ directory in the `libxdp`_ repository.
+For a detailed explaination of the AF_XDP interface please see:
+
+- `libxdp-readme`_.
+- `AF_XDP`_ kernel documentation.
+
+.. note::
+    The most comprehensive resource for using XSKMAPs and AF_XDP is `libxdp`_.
+
+.. _libxdp: https://github.com/xdp-project/xdp-tools/tree/master/lib/libxdp
+.. _AF_XDP: https://www.kernel.org/doc/html/latest/networking/af_xdp.html
+.. _bpf-examples: https://github.com/xdp-project/bpf-examples
+.. _libxdp-readme: https://github.com/xdp-project/xdp-tools/tree/master/lib/libxdp#using-af_xdp-sockets