[PATCH bpf-next v2 1/1] doc: DEVMAPs and XDP_REDIRECT

[mtahhan@redhat.com]

From: Maryam Tahhan <mtahhan@redhat.com>

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 <mtahhan@redhat.com>
---
 Documentation/bpf/index.rst      |   1 +
 Documentation/bpf/map_devmap.rst | 192 +++++++++++++++++++++++++++++++
 Documentation/bpf/redirect.rst   |  46 ++++++++
 3 files changed, 239 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..bdba55640f4c
--- /dev/null
+++ b/Documentation/bpf/map_devmap.rst
@@ -0,0 +1,192 @@
+.. 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:
+
+ - 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.
+
+.. 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
-- 
2.35.3