On Tue, Nov 22, 2022 at 10:37:01AM +0000, mtahhan@xxxxxxxxxx wrote:
> From: Maryam Tahhan <mtahhan@xxxxxxxxxx>
>
> Add documentation for BPF_MAP_TYPE_XSKMAP
> including kernel version introduced, usage
> and examples.
>
> Signed-off-by: Maryam Tahhan <mtahhan@xxxxxxxxxx>
>
> ---
> v4:
> - Add more details about AF_XDP sockets and where to find
> relevant info.
> - Fixup typos.
> - Remove ``c:function::`` block directives.
> - Replace spaces with tabs in code blocks.
>
> v3:
> - Fixed duplicate function warnings from Sphinx >= 3.3.
>
> 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 | 192 +++++++++++++++++++++++++++++++
> 1 file changed, 192 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..990010952d2f
> --- /dev/null
> +++ b/Documentation/bpf/map_xskmap.rst
> @@ -0,0 +1,192 @@
> +.. 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
s/Kernel/kernel
> +userspace without having to traverse the full network stack. An AF_XDP socket
We make this mistake all over the BPF docs, but we might as well be
consistent / correct here:
s/userspace/user space
> +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
s/Userspace/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.
> +
> +Userspace
> +---------
s/Userspace/User space
> +.. note::
> + XSK entries can only be updated/deleted from user space and not from
> + an BPF program. Trying to call these functions from a kernel BPF program will
s/an BPF/a BPF
> + 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;
> + }
> +
> +Userspace
> +---------
s/Userspace/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
> --
> 2.34.1
>
