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Date:   Mon, 12 Dec 2022 18:35:51 -0800
From:   Stanislav Fomichev <>
        David Ahern <>,
        Jakub Kicinski <>,
        Willem de Bruijn <>,
        Jesper Dangaard Brouer <>,
        Anatoly Burakov <>,
        Alexander Lobakin <>,
        Magnus Karlsson <>,
        Maryam Tahhan <>,,
Subject: [PATCH bpf-next v4 01/15] bpf: Document XDP RX metadata

Document all current use-cases and assumptions.

Cc: John Fastabend <>
Cc: David Ahern <>
Cc: Martin KaFai Lau <>
Cc: Jakub Kicinski <>
Cc: Willem de Bruijn <>
Cc: Jesper Dangaard Brouer <>
Cc: Anatoly Burakov <>
Cc: Alexander Lobakin <>
Cc: Magnus Karlsson <>
Cc: Maryam Tahhan <>
Signed-off-by: Stanislav Fomichev <>
 Documentation/bpf/xdp-rx-metadata.rst | 90 +++++++++++++++++++++++++++
 1 file changed, 90 insertions(+)
 create mode 100644 Documentation/bpf/xdp-rx-metadata.rst

diff --git a/Documentation/bpf/xdp-rx-metadata.rst b/Documentation/bpf/xdp-rx-metadata.rst
new file mode 100644
index 000000000000..498eae718275
--- /dev/null
+++ b/Documentation/bpf/xdp-rx-metadata.rst
@@ -0,0 +1,90 @@
+XDP RX Metadata
+XDP programs support creating and passing custom metadata via
+``bpf_xdp_adjust_meta``. This metadata can be consumed by the following
+1. ``AF_XDP`` consumer.
+2. Kernel core stack via ``XDP_PASS``.
+3. Another device via ``bpf_redirect_map``.
+4. Other BPF programs via ``bpf_tail_call``.
+General Design
+XDP has access to a set of kfuncs to manipulate the metadata. Every
+device driver implements these kfuncs. The set of kfuncs is
+declared in ``include/net/xdp.h`` via ``XDP_METADATA_KFUNC_xxx``.
+Currently, the following kfuncs are supported. In the future, as more
+metadata is supported, this set will grow:
+- ``bpf_xdp_metadata_rx_timestamp_supported`` returns true/false to
+  indicate whether the device supports RX timestamps
+- ``bpf_xdp_metadata_rx_timestamp`` returns packet RX timestamp
+- ``bpf_xdp_metadata_rx_hash_supported`` returns true/false to
+  indicate whether the device supports RX hash
+- ``bpf_xdp_metadata_rx_hash`` returns packet RX hash
+Within the XDP frame, the metadata layout is as follows::
+  +----------+-----------------+------+
+  | headroom | custom metadata | data |
+  +----------+-----------------+------+
+             ^                 ^
+             |                 |
+   xdp_buff->data_meta   xdp_buff->data
+``AF_XDP`` use-case implies that there is a contract between the BPF program
+that redirects XDP frames into the ``XSK`` and the final consumer.
+Thus the BPF program manually allocates a fixed number of
+bytes out of metadata via ``bpf_xdp_adjust_meta`` and calls a subset
+of kfuncs to populate it. User-space ``XSK`` consumer, looks
+at ``xsk_umem__get_data() - METADATA_SIZE`` to locate its metadata.
+Here is the ``AF_XDP`` consumer layout (note missing ``data_meta`` pointer)::
+  +----------+-----------------+------+
+  | headroom | custom metadata | data |
+  +----------+-----------------+------+
+                               ^
+                               |
+                        rx_desc->address
+This is the path where the packets processed by the XDP program are passed
+into the kernel. The kernel creates ``skb`` out of the ``xdp_buff`` contents.
+Currently, every driver has a custom kernel code to parse the descriptors and
+populate ``skb`` metadata when doing this ``xdp_buff->skb`` conversion.
+In the future, we'd like to support a case where XDP program can override
+some of that metadata.
+The plan of record is to make this path similar to ``bpf_redirect_map``
+so the program can control which metadata is passed to the skb layer.
+``bpf_redirect_map`` can redirect the frame to a different device.
+In this case we don't know ahead of time whether that final consumer
+will further redirect to an ``XSK`` or pass it to the kernel via ``XDP_PASS``.
+Additionally, the final consumer doesn't have access to the original
+hardware descriptor and can't access any of the original metadata.
+For this use-case, only custom metadata is currently supported. If
+the frame is eventually passed to the kernel, the skb created from such
+a frame won't have any skb metadata. The ``XSK`` consumer will only
+have access to the custom metadata.
+No special handling here. Tail-called program operates on the same context
+as the original one.

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