- how to use
- how to query features
- pointers to the examples
Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
---
Documentation/networking/index.rst | 1 +
Documentation/networking/xsk-tx-metadata.rst | 75 ++++++++++++++++++++
2 files changed, 76 insertions(+)
create mode 100644 Documentation/networking/xsk-tx-metadata.rst
diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index 5b75c3f7a137..9b2accb48df7 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -123,6 +123,7 @@ Refer to :ref:`netdev-FAQ` for a guide on netdev development process specifics.
xfrm_sync
xfrm_sysctl
xdp-rx-metadata
+ xsk-tx-metadata
.. only:: subproject and html
diff --git a/Documentation/networking/xsk-tx-metadata.rst b/Documentation/networking/xsk-tx-metadata.rst
new file mode 100644
index 000000000000..41600f08f794
--- /dev/null
+++ b/Documentation/networking/xsk-tx-metadata.rst
@@ -0,0 +1,75 @@
+==================
+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 ``setsockopt(fd, SOL_XDP,
+XDP_TX_METADATA_LEN, &len, 4)``. The metadata layout is a fixed UAPI,
+refer to ``struct xsk_tx_metadata`` in ``include/uapi/linux/if_xdp.h``.
+IOW, the ``len`` variable above should contain
+``sizeof(struct 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::
+
+ XDP_TX_METADATA_LEN
+ / \
+ +-----------------+---------+----------------------------+
+ | xsk_tx_metadata | padding | payload |
+ +-----------------+---------+----------------------------+
+ ^
+ |
+ xdp_desc->addr
+
+An AF_XDP applications can request headrooms larger than ``sizeof(struct
+xsk_tx_metadata)``. The kernel will ignore the padding (and will still
+use ``xdp_desc->addr - XDP_TX_METADATA_LEN`` to locate
+the ``xsk_tx_metadata``). The application is expected to zero-out
+the metadata flags for the frames that shouldn't use any offloads.
+
+The flags field enables the particular offload:
+
+- ``XDP_TX_METADATA_TIMESTAMP``: requests the device to put transmission
+ timestamp into ``tx_timestamp`` field of ``struct xsk_tx_metadata``.
+- ``XDP_TX_METADATA_CHECKSUM``: requests the device to calculate L4
+ checksum. ``csum_start`` specifies byte offset of there the checksumming
+ should start and ``csum_offset`` specifies byte offset where the
+ device should store the computed checksum.
+- ``XDP_TX_METADATA_CHECKSUM_SW``: requests checksum calculation to
+ be done in software; this mode works only in ``XSK_COPY`` mode and
+ is mostly intended for testing. Do not enable this option, it
+ will negatively affect performance.
+
+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 not that in a multi-buffer packet
+only the first chunk should carry the metadata.
+
+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_TX_METADATA_TIMESTAMP``
+- ``tx-checksum``: device supports ``XDP_TX_METADATA_CHECKSUM``
+
+Note that every devices supports ``XDP_TX_METADATA_CHECKSUM_SW`` when
+running in ``XSK_COPY`` mode.
+
+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.
--
2.41.0.640.ga95def55d0-goog
