diff options
Diffstat (limited to 'Documentation/networking/xdp-rx-metadata.rst')
| -rw-r--r-- | Documentation/networking/xdp-rx-metadata.rst | 50 |
1 files changed, 49 insertions, 1 deletions
diff --git a/Documentation/networking/xdp-rx-metadata.rst b/Documentation/networking/xdp-rx-metadata.rst index 25ce72af81c2..ce96f4c99505 100644 --- a/Documentation/networking/xdp-rx-metadata.rst +++ b/Documentation/networking/xdp-rx-metadata.rst @@ -1,3 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + =============== XDP RX Metadata =============== @@ -18,7 +20,13 @@ Currently, the following kfuncs are supported. In the future, as more metadata is supported, this set will grow: .. kernel-doc:: net/core/xdp.c - :identifiers: bpf_xdp_metadata_rx_timestamp bpf_xdp_metadata_rx_hash + :identifiers: bpf_xdp_metadata_rx_timestamp + +.. kernel-doc:: net/core/xdp.c + :identifiers: bpf_xdp_metadata_rx_hash + +.. kernel-doc:: net/core/xdp.c + :identifiers: bpf_xdp_metadata_rx_vlan_tag An XDP program can use these kfuncs to read the metadata into stack variables for its own consumption. Or, to pass the metadata on to other @@ -105,6 +113,46 @@ bpf_tail_call Adding programs that access metadata kfuncs to the ``BPF_MAP_TYPE_PROG_ARRAY`` is currently not supported. +Supported Devices +================= + +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 ======= |