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+.. SPDX-License-Identifier: GPL-2.0
+
+==================
+AF_XDP TX Metadata
+==================
+
+This document describes how to enable offloads when transmitting packets
+via :doc:`af_xdp`. Refer to :doc:`xdp-rx-metadata` on how to access similar
+metadata on the receive side.
+
+General Design
+==============
+
+The headroom for the metadata is reserved via ``tx_metadata_len`` in
+``struct xdp_umem_reg``. The metadata length is therefore the same for
+every socket that shares the same umem. The metadata layout is a fixed UAPI,
+refer to ``union xsk_tx_metadata`` in ``include/uapi/linux/if_xdp.h``.
+Thus, generally, the ``tx_metadata_len`` field above should contain
+``sizeof(union xsk_tx_metadata)``.
+
+The headroom and the metadata itself should be located right before
+``xdp_desc->addr`` in the umem frame. Within a frame, the metadata
+layout is as follows::
+
+ tx_metadata_len
+ / \
+ +-----------------+---------+----------------------------+
+ | xsk_tx_metadata | padding | payload |
+ +-----------------+---------+----------------------------+
+ ^
+ |
+ xdp_desc->addr
+
+An AF_XDP application can request headrooms larger than ``sizeof(struct
+xsk_tx_metadata)``. The kernel will ignore the padding (and will still
+use ``xdp_desc->addr - tx_metadata_len`` to locate
+the ``xsk_tx_metadata``). For the frames that shouldn't carry
+any metadata (i.e., the ones that don't have ``XDP_TX_METADATA`` option),
+the metadata area is ignored by the kernel as well.
+
+The flags field enables the particular offload:
+
+- ``XDP_TXMD_FLAGS_TIMESTAMP``: requests the device to put transmission
+ timestamp into ``tx_timestamp`` field of ``union xsk_tx_metadata``.
+- ``XDP_TXMD_FLAGS_CHECKSUM``: requests the device to calculate L4
+ checksum. ``csum_start`` specifies byte offset of where the checksumming
+ should start and ``csum_offset`` specifies byte offset where the
+ device should store the computed checksum.
+
+Besides the flags above, in order to trigger the offloads, the first
+packet's ``struct xdp_desc`` descriptor should set ``XDP_TX_METADATA``
+bit in the ``options`` field. Also note that in a multi-buffer packet
+only the first chunk should carry the metadata.
+
+Software TX Checksum
+====================
+
+For development and testing purposes its possible to pass
+``XDP_UMEM_TX_SW_CSUM`` flag to ``XDP_UMEM_REG`` UMEM registration call.
+In this case, when running in ``XDK_COPY`` mode, the TX checksum
+is calculated on the CPU. Do not enable this option in production because
+it will negatively affect performance.
+
+Querying Device Capabilities
+============================
+
+Every devices exports its offloads capabilities via netlink netdev family.
+Refer to ``xsk-flags`` features bitmask in
+``Documentation/netlink/specs/netdev.yaml``.
+
+- ``tx-timestamp``: device supports ``XDP_TXMD_FLAGS_TIMESTAMP``
+- ``tx-checksum``: device supports ``XDP_TXMD_FLAGS_CHECKSUM``
+
+See ``tools/net/ynl/samples/netdev.c`` on how to query this information.
+
+Example
+=======
+
+See ``tools/testing/selftests/bpf/xdp_hw_metadata.c`` for an example
+program that handles TX metadata. Also see https://github.com/fomichev/xskgen
+for a more bare-bones example.