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Diffstat (limited to 'Documentation/networking/xdp-rx-metadata.rst')
| -rw-r--r-- | Documentation/networking/xdp-rx-metadata.rst | 33 |
1 files changed, 33 insertions, 0 deletions
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 ======= |