From: Maryam Tahhan <mtahhan@xxxxxxxxxx> Add documentation for BPF_MAP_TYPE_DEVMAP and BPF_MAP_TYPE_DEVMAP_HASH including kernel version introduced, usage and examples. Signed-off-by: Maryam Tahhan <mtahhan@xxxxxxxxxx> --- Documentation/bpf/map_devmap.rst | 231 +++++++++++++++++++++++++++++++ 1 file changed, 231 insertions(+) create mode 100644 Documentation/bpf/map_devmap.rst diff --git a/Documentation/bpf/map_devmap.rst b/Documentation/bpf/map_devmap.rst new file mode 100644 index 000000000000..ecd2b7b951cc --- /dev/null +++ b/Documentation/bpf/map_devmap.rst @@ -0,0 +1,231 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2022 Red Hat, Inc. + +================================================= +BPF_MAP_TYPE_DEVMAP and BPF_MAP_TYPE_DEVMAP_HASH +================================================= + +.. note:: + - ``BPF_MAP_TYPE_DEVMAP`` was introduced in kernel version 4.14 + - ``BPF_MAP_TYPE_DEVMAP_HASH`` was introduced in kernel version 5.4 + +``BPF_MAP_TYPE_DEVMAP`` is a BPF map, primarily used as a backend map for the XDP +BPF helper call ``bpf_redirect_map()``. It's backed by an array that uses the key as +the index to lookup a reference to a net device. The user provides <``key``/ ``ifindex``> +pairs to update the map with new net devices. + +``BPF_MAP_TYPE_DEVMAP_HASH`` is also a backend map for ``bpf_redirect_map()``. +It's backed by a hash table that uses the ``ifindex`` as the key to lookup a reference +to a net device. As it's a hash map, it allows for densely packing the net devices +(compared with the sparsely packed ``BPF_MAP_TYPE_DEVMAP``). The user provides +<``key``/ ``struct bpf_devmap_val``> pairs to update the map with new net devices. + +The setup and packet enqueue/send code is shared between the two types of +devmap; only the lookup and insertion is different. + +XDP_REDIRECT +============ + +XDP_REDIRECT works by a three-step process, implemented as follows: + +1. The ``bpf_redirect()`` and ``bpf_redirect_map()`` helpers will lookup the + target of the redirect and store it (along with some other metadata) in a + per-CPU ``struct bpf_redirect_info``. This is where the maps above come into + play. + +2. When the program returns the ``XDP_REDIRECT`` return code, the driver will + call ``xdp_do_redirect()`` which will use the information in ``struct + bpf_redirect_info`` to actually enqueue the frame into a map type-specific + bulk queue structure. + +3. Before exiting its NAPI poll loop, the driver will call ``xdp_do_flush()``, + which will flush all the different bulk queues, thus completing the + redirect. + +Pointers to the map entries will be kept around for this whole sequence of +steps, protected by RCU. However, there is no top-level ``rcu_read_lock()`` in +the core code; instead, the RCU protection relies on everything happening +inside a single NAPI poll sequence, which means it's between a pair of calls +to ``local_bh_disable()`` / ``local_bh_enable()``. + +The map entries are marked as ``__rcu`` and the map code makes sure to dereference +those pointers with ``rcu_dereference_check()`` in a way that works for both +sections that to hold an ``rcu_read_lock()`` and sections that are called from +NAPI without a separate ``rcu_read_lock()``. The code below does not use RCU +annotations, but relies on those in the map code. + +.. note:: + ``XDP_REDIRECT`` is not fully supported yet for xdp frags since not all XDP + capable drivers can map a non-linear ``xdp_frame`` in ``ndo_xdp_xmit``. + +Usage +===== + +.. c:function:: + long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags) + + Net device entries can be added or updated using the ``bpf_map_update_elem()`` + helper. This helper replaces existing elements atomically. The ``flags`` + parameter can be used to control the update behaviour: + + - ``BPF_ANY`` will create a new element or update an existing element + - ``BPF_NOEXIST`` will create a new element only if one did not already + exist + - ``BPF_EXIST`` will update an existing element + + ``bpf_map_update_elem()`` returns 0 on success, or negative error in + case of failure. + + The value parameter is of type ``struct bpf_devmap_val``: + + .. code-block:: c + + struct bpf_devmap_val { + __u32 ifindex; /* device index */ + union { + int fd; /* prog fd on map write */ + __u32 id; /* prog id on map read */ + } bpf_prog; + }; + + DEVMAPs can associate a program with a device entry by adding a ``bpf_prog.fd`` + to ``struct bpf_devmap_val``. Programs are run after ``XDP_REDIRECT`` and have + access to both Rx device and Tx device. The program associated with the ``fd`` + must have type XDP with expected attach type ``xdp_devmap``. + When a program is associated with a device index, the program is run on an + ``XDP_REDIRECT`` and before the buffer is added to the per-cpu queue. Examples + of how to attach/use xdp_devmap progs can be found in the kernel selftests: + + - test_xdp_with_devmap_helpers_ + - xdp_devmap_attach_ + +.. _xdp_devmap_attach: https://github.com/torvalds/linux/blob/master/tools/testing/selftests/bpf/prog_tests/xdp_devmap_attach.c +.. _test_xdp_with_devmap_helpers: https://github.com/torvalds/linux/blob/master/tools/testing/selftests/bpf/progs/test_xdp_with_devmap_helpers.c + +.. c:function:: + void *bpf_map_lookup_elem(struct bpf_map *map, const void *key) + + net device entries can be retrieved using the ``bpf_map_lookup_elem()`` + helper. This helper returns a pointer to the value associated with ``key``, or + ``NULL`` if no entry was found. + +.. c:function:: + long bpf_map_delete_elem(struct bpf_map *map, const void *key) + + net device 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. + +.. 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_DEVMAP`` and ``BPF_MAP_TYPE_DEVMAP_HASH`` this map contains + references to net devices (for forwarding packets through other ports). + + The lower two bits of *flags* are used as the return code if the map lookup + fails. This is so that the return value can be one of the XDP program return + codes up to ``XDP_TX``, as chosen by the caller. The higher bits of ``flags`` + can be set to ``BPF_F_BROADCAST`` or ``BPF_F_EXCLUDE_INGRESS`` as defined + below. + + With ``BPF_F_BROADCAST`` the packet will be broadcast to all the interfaces + in the map, with ``BPF_F_EXCLUDE_INGRESS`` the ingress interface will be excluded + from the broadcast. + + This helper will return ``XDP_REDIRECT`` on success, or the value of the two + lower bits of the *flags* argument if the map lookup fails. + +Examples +======== + +Kernel BPF +---------- + +The following code snippet shows how to declare a ``BPF_MAP_TYPE_DEVMAP`` +called tx_port. + +.. code-block:: c + + struct { + __uint(type, BPF_MAP_TYPE_DEVMAP); + __uint(key_size, sizeof(int)); + __uint(value_size, sizeof(int)); + __uint(max_entries, 256); + } tx_port SEC(".maps"); + +The following code snippet shows how to declare a ``BPF_MAP_TYPE_DEVMAP_HASH`` +called forward_map. + +.. code-block:: c + + struct { + __uint(type, BPF_MAP_TYPE_DEVMAP_HASH); + __uint(key_size, sizeof(int)); + __uint(value_size, sizeof(struct bpf_devmap_val)); + __uint(max_entries, 32); + } forward_map SEC(".maps"); + +The following code snippet shows a simple xdp_redirect_map program. + +.. code-block:: c + + SEC("xdp") + int xdp_redirect_map_func(struct xdp_md *ctx) + { + void *data_end = (void *)(long)ctx->data_end; + void *data = (void *)(long)ctx->data; + int action = XDP_PASS; + int index = ctx->ingress_ifindex; + + action = bpf_redirect_map(&tx_port, index, BPF_F_BROADCAST | BPF_F_EXCLUDE_INGRESS); + + out: + return action; + } + + +User space +---------- + +The following code snippet shows how to update a devmap called ``tx_port``. + +.. code-block:: c + + int update_devmap(int ifindex, int redirect_ifindex) + { + int ret = -1; + + ret = bpf_map_update_elem(bpf_map__fd(tx_port), &ifindex, &redirect_ifindex, 0); + if (ret < 0) { + fprintf(stderr, "Failed to update devmap_ value: %s\n", + strerror(errno)); + } + + return ret; + } + +The following code snippet shows how to update a hash_devmap called ``forward_map``. + +.. code-block:: c + + int update_devmap(int ifindex, int redirect_ifindex) + { + struct bpf_devmap_val devmap_val; + int ret = -1; + + devmap_val.ifindex = redirect_ifindex; + ret = bpf_map_update_elem(bpf_map__fd(forward_map), &ifindex, &devmap_val, 0); + if (ret < 0) { + fprintf(stderr, "Failed to update devmap_ value: %s\n", + strerror(errno)); + } + return ret; + } + +References +=========== + +- https://lwn.net/Articles/728146/ +- https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=6f9d451ab1a33728adb72d7ff66a7b374d665176 +- https://elixir.bootlin.com/linux/latest/source/net/core/filter.c#L4106 -- 2.35.3