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[66.187.232.132]) by smtp.gmail.com with ESMTPSA id a20-20020a05622a065400b0039853b7b771sm7571591qtb.80.2022.10.17.01.52.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 Oct 2022 01:52:34 -0700 (PDT) From: mtahhan@redhat.com To: bpf@vger.kernel.org, linux-doc@vger.kernel.org Cc: Maryam Tahhan Subject: [PATCH bpf-next v3 1/1] doc: DEVMAPs and XDP_REDIRECT Date: Mon, 17 Oct 2022 05:47:53 -0400 Message-Id: <20221017094753.1564273-2-mtahhan@redhat.com> X-Mailer: git-send-email 2.35.3 In-Reply-To: <20221017094753.1564273-1-mtahhan@redhat.com> References: <20221017094753.1564273-1-mtahhan@redhat.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org X-Patchwork-Delegate: bpf@iogearbox.net From: Maryam Tahhan Add documentation for BPF_MAP_TYPE_DEVMAP and BPF_MAP_TYPE_DEVMAP_HASH including kernel version introduced, usage and examples. Add documentation that describes XDP_REDIRECT. Signed-off-by: Maryam Tahhan --- Documentation/bpf/index.rst | 1 + Documentation/bpf/map_devmap.rst | 189 +++++++++++++++++++++++++++++++ Documentation/bpf/redirect.rst | 46 ++++++++ 3 files changed, 236 insertions(+) create mode 100644 Documentation/bpf/map_devmap.rst create mode 100644 Documentation/bpf/redirect.rst diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst index 1b50de1983ee..1088d44634d6 100644 --- a/Documentation/bpf/index.rst +++ b/Documentation/bpf/index.rst @@ -29,6 +29,7 @@ that goes into great technical depth about the BPF Architecture. clang-notes linux-notes other + redirect .. only:: subproject and html diff --git a/Documentation/bpf/map_devmap.rst b/Documentation/bpf/map_devmap.rst new file mode 100644 index 000000000000..57997cb2b619 --- /dev/null +++ b/Documentation/bpf/map_devmap.rst @@ -0,0 +1,189 @@ +.. 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`` and ``BPF_MAP_TYPE_DEVMAP_HASH`` are BPF maps primarily +used as backend maps for the XDP BPF helper call ``bpf_redirect_map()``. +``BPF_MAP_TYPE_DEVMAP`` is backed by an array that uses the key as +the index to lookup a reference to a net device. While ``BPF_MAP_TYPE_DEVMAP_HASH`` +is backed by a hash table that uses the ``ifindex`` as the key to lookup a reference +to a net device. The user provides either <``key``/ ``ifindex``> or +<``key``/ ``struct bpf_devmap_val``> pairs to update the maps with new net devices. + +.. note:: + While ``BPF_MAP_TYPE_DEVMAP_HASH`` allows for densely packing the net devices + it comes at the cost of a hash of the key when performing a look up. + +The setup and packet enqueue/send code is shared between the two types of +devmap; only the lookup and insertion is different. + +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 ``value`` parameter + can be ``struct bpf_devmap_val`` or a simple ``int ifindex`` for backwards + compatibility. + +.. note:: + The maps can only be updated from user space and not from a BPF program. + +.. 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: + +- /tools/testing/selftests/bpf/prog_tests/xdp_devmap_attach.c +- /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. + +.. 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. + +More information about redirection can be found :doc:`redirect` + +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); + __type(key, __u32); + __type(value, __u32); + __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); + __type(key, __u32); + __type(value, struct bpf_devmap_val); + __uint(max_entries, 32); + } forward_map SEC(".maps"); + +.. note:: + + The value type in the DEVMAP above is a ``struct bpf_devmap_val`` + +The following code snippet shows a simple xdp_redirect_map program. This program +would work with a user space program that populates the devmap ``tx_port`` based +on ingress ifindexes. The BPF program (below) is redirecting packets using the +ingress ifindex as the ``key``. + +.. code-block:: c + + SEC("xdp") + int xdp_redirect_map_func(struct xdp_md *ctx) + { + int index = ctx->ingress_ifindex; + + return bpf_redirect_map(&tx_port, index, BPF_F_BROADCAST | BPF_F_EXCLUDE_INGRESS); + } + + +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 = { .ifindex = redirect_ifindex }; + int ret = -1; + + 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 diff --git a/Documentation/bpf/redirect.rst b/Documentation/bpf/redirect.rst new file mode 100644 index 000000000000..5a0377a67ff0 --- /dev/null +++ b/Documentation/bpf/redirect.rst @@ -0,0 +1,46 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2022 Red Hat, Inc. + +============ +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. + +.. note:: + Not all drivers support transmitting frames after a redirect, and for + those that do, not all of them support non-linear frames. Non-linear xdp + bufs/frames are bufs/frames that contain more than one fragment. + +XDP_REDIRECT works with the following map types: + +- BPF_MAP_TYPE_DEVMAP +- BPF_MAP_TYPE_DEVMAP_HASH +- BPF_MAP_TYPE_CPUMAP +- BPF_MAP_TYPE_XSKMAP + +For more information on these maps, please see the specific map documentation. + +References +=========== + +- https://elixir.bootlin.com/linux/latest/source/net/core/filter.c#L4106