1.. SPDX-License-Identifier: GPL-2.0
2
3==================
4AF_XDP TX Metadata
5==================
6
7This document describes how to enable offloads when transmitting packets
8via :doc:`af_xdp`. Refer to :doc:`xdp-rx-metadata` on how to access similar
9metadata on the receive side.
10
11General Design
12==============
13
14The headroom for the metadata is reserved via ``tx_metadata_len`` and
15``XDP_UMEM_TX_METADATA_LEN`` flag in ``struct xdp_umem_reg``. The metadata
16length is therefore the same for every socket that shares the same umem.
17The metadata layout is a fixed UAPI, refer to ``union xsk_tx_metadata`` in
18``include/uapi/linux/if_xdp.h``. Thus, generally, the ``tx_metadata_len``
19field above should contain ``sizeof(union xsk_tx_metadata)``.
20
21Note that in the original implementation the ``XDP_UMEM_TX_METADATA_LEN``
22flag was not required. Applications might attempt to create a umem
23with a flag first and if it fails, do another attempt without a flag.
24
25The headroom and the metadata itself should be located right before
26``xdp_desc->addr`` in the umem frame. Within a frame, the metadata
27layout is as follows::
28
29           tx_metadata_len
30     /                         \
31    +-----------------+---------+----------------------------+
32    | xsk_tx_metadata | padding |          payload           |
33    +-----------------+---------+----------------------------+
34                                ^
35                                |
36                          xdp_desc->addr
37
38An AF_XDP application can request headrooms larger than ``sizeof(struct
39xsk_tx_metadata)``. The kernel will ignore the padding (and will still
40use ``xdp_desc->addr - tx_metadata_len`` to locate
41the ``xsk_tx_metadata``). For the frames that shouldn't carry
42any metadata (i.e., the ones that don't have ``XDP_TX_METADATA`` option),
43the metadata area is ignored by the kernel as well.
44
45The flags field enables the particular offload:
46
47- ``XDP_TXMD_FLAGS_TIMESTAMP``: requests the device to put transmission
48  timestamp into ``tx_timestamp`` field of ``union xsk_tx_metadata``.
49- ``XDP_TXMD_FLAGS_CHECKSUM``: requests the device to calculate L4
50  checksum. ``csum_start`` specifies byte offset of where the checksumming
51  should start and ``csum_offset`` specifies byte offset where the
52  device should store the computed checksum.
53
54Besides the flags above, in order to trigger the offloads, the first
55packet's ``struct xdp_desc`` descriptor should set ``XDP_TX_METADATA``
56bit in the ``options`` field. Also note that in a multi-buffer packet
57only the first chunk should carry the metadata.
58
59Software TX Checksum
60====================
61
62For development and testing purposes its possible to pass
63``XDP_UMEM_TX_SW_CSUM`` flag to ``XDP_UMEM_REG`` UMEM registration call.
64In this case, when running in ``XDK_COPY`` mode, the TX checksum
65is calculated on the CPU. Do not enable this option in production because
66it will negatively affect performance.
67
68Querying Device Capabilities
69============================
70
71Every devices exports its offloads capabilities via netlink netdev family.
72Refer to ``xsk-flags`` features bitmask in
73``Documentation/netlink/specs/netdev.yaml``.
74
75- ``tx-timestamp``: device supports ``XDP_TXMD_FLAGS_TIMESTAMP``
76- ``tx-checksum``: device supports ``XDP_TXMD_FLAGS_CHECKSUM``
77
78See ``tools/net/ynl/samples/netdev.c`` on how to query this information.
79
80Example
81=======
82
83See ``tools/testing/selftests/bpf/xdp_hw_metadata.c`` for an example
84program that handles TX metadata. Also see https://github.com/fomichev/xskgen
85for a more bare-bones example.
86