Re: [PATCH bpf-next v3 1/1] doc: DEVMAPs and XDP_REDIRECT

[Donald Hunter <donald.hunter@gmail.com>]

mtahhan@redhat.com 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