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