[PATCH bpf-next,v5 1/1] doc: xdp: clarify driver implementation for XDP Rx metadata

[Song Yoong Siang <yoong.siang.song@intel.com>]

Clarify that drivers must remove device-reserved metadata from the
data_meta area before passing frames to XDP programs.

Additionally, expand the explanation of how userspace and BPF programs
should coordinate the use of METADATA_SIZE, and add a detailed diagram
to illustrate pointer adjustments and metadata layout.

Also describe the requirements and constraints enforced by
bpf_xdp_adjust_meta().

Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
---
V5:
  - create a new section called 'Driver implementation' (Stanislav)
  - reword 'utilize the data_meta area' to 'prepend metadata to received packets' (Jakub)

V4: https://lore.kernel.org/netdev/20250715071502.3503440-1-yoong.siang.song@intel.com/
  - update the documentation to indicate that drivers are expected to copy
    any device-reserved metadata from the metadata area (Jakub)
  - remove selftest tool changes.

V3: https://lore.kernel.org/netdev/20250702165757.3278625-1-yoong.siang.song@intel.com/
  - update doc and commit msg accordingly.

V2: https://lore.kernel.org/netdev/20250702030349.3275368-1-yoong.siang.song@intel.com/
  - unconditionally do bpf_xdp_adjust_meta with -XDP_METADATA_SIZE (Stanislav)

V1: https://lore.kernel.org/netdev/20250701042940.3272325-1-yoong.siang.song@intel.com/
---
 Documentation/networking/xdp-rx-metadata.rst | 33 ++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/Documentation/networking/xdp-rx-metadata.rst b/Documentation/networking/xdp-rx-metadata.rst
index a6e0ece18be5..ce96f4c99505 100644
--- a/Documentation/networking/xdp-rx-metadata.rst
+++ b/Documentation/networking/xdp-rx-metadata.rst
@@ -120,6 +120,39 @@ It is possible to query which kfunc the particular netdev implements via
 netlink. See ``xdp-rx-metadata-features`` attribute set in
 ``Documentation/netlink/specs/netdev.yaml``.
 
+Driver Implementation
+=====================
+
+Certain devices may prepend metadata to received packets. However, as of now,
+``AF_XDP`` lacks the ability to communicate the size of the ``data_meta`` area
+to the consumer. Therefore, it is the responsibility of the driver to copy any
+device-reserved metadata out from the metadata area and ensure that
+``xdp_buff->data_meta`` is pointing to ``xdp_buff->data`` before presenting the
+frame to the XDP program. This is necessary so that, after the XDP program
+adjusts the metadata area, the consumer can reliably retrieve the metadata
+address using ``METADATA_SIZE`` offset.
+
+The following diagram shows how custom metadata is positioned relative to the
+packet data and how pointers are adjusted for metadata access::
+
+              |<-- bpf_xdp_adjust_meta(xdp_buff, -METADATA_SIZE) --|
+  new xdp_buff->data_meta                              old xdp_buff->data_meta
+              |                                                    |
+              |                                            xdp_buff->data
+              |                                                    |
+   +----------+----------------------------------------------------+------+
+   | headroom |                  custom metadata                   | data |
+   +----------+----------------------------------------------------+------+
+              |                                                    |
+              |                                            xdp_desc->addr
+              |<------ xsk_umem__get_data() - METADATA_SIZE -------|
+
+``bpf_xdp_adjust_meta`` ensures that ``METADATA_SIZE`` is aligned to 4 bytes,
+does not exceed 252 bytes, and leaves sufficient space for building the
+xdp_frame. If these conditions are not met, it returns a negative error. In this
+case, the BPF program should not proceed to populate data into the ``data_meta``
+area.
+
 Example
 =======
 
-- 
2.34.1