mtahhan@xxxxxxxxxx writes: > 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. Can you remove the last sentence above. Instead, maybe mention that the lookup happens in the provided map which has to be one of the supported map types. > +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
