[PATCH bpf-next v2 1/1] docs: BPF_MAP_TYPE_XSKMAP

[mtahhan@redhat.com]

From: Maryam Tahhan <mtahhan@redhat.com>

Add documentation for BPF_MAP_TYPE_XSKMAP
including kernel version introduced, usage
and examples.

Signed-off-by: Maryam Tahhan <mtahhan@redhat.com>

---
v2:
- Fixed typos + incorrect return type references.
- Adjusted examples to use __u32 and fixed references to key_size.
- Changed `AF_XDP socket` references to XSK.
- Added note re map key and value size.
---
 Documentation/bpf/map_xskmap.rst | 167 +++++++++++++++++++++++++++++++
 1 file changed, 167 insertions(+)
 create mode 100644 Documentation/bpf/map_xskmap.rst

diff --git a/Documentation/bpf/map_xskmap.rst b/Documentation/bpf/map_xskmap.rst
new file mode 100644
index 000000000000..4fd2f58aa629
--- /dev/null
+++ b/Documentation/bpf/map_xskmap.rst
@@ -0,0 +1,167 @@
+.. 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). 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     |<---+ Userspace
+    =========================================================|==========
+    |    Queue 0     |     Queue 1     |     Queue 2    |    |  Kernel
+    +---------------------------------------------------+    |
+    |                  Netdev eth0                      |    |
+    +---------------------------------------------------+    |
+    |                            +=============+        |    |
+    |                            | key |  xsk  |        |    |
+    |  +---------+               +=============+        |    |
+    |  |         |               |  0  | xsk A |        |    |
+    |  |         |               +-------------+        |    |
+    |  |         |               |  1  | xsk B |        |    |
+    |  | eBPF    |-- 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 a 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
+----------
+.. c:function::
+     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.
+
+.. c:function::
+    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.
+
+Userspace
+---------
+.. note::
+    XSK entries can only be updated/deleted from user space and not from
+    an eBPF program. Trying to call these functions from a kernel eBPF program will
+    result in the program failing to load and a verifier warning.
+
+.. c:function::
+	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.
+
+.. c:function::
+    int bpf_map_lookup_elem(int fd, const void *key, void *value)
+
+  Returns ``struct xdp_sock *`` or negative error in case of failure.
+
+.. c:function::
+    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 a 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 a 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;
+    }
+
+Userspace
+---------
+
+The following code snippet shows how to update a XSKMAP with a 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;
+    }
+
+.. note::
+    The most comprehensive resource for using XSKMAPs is `libxdp`_.
+
+.. _libxdp: https://github.com/xdp-project/xdp-tools/tree/master/lib/libxdp
-- 
2.38.1