[xdp-hints] [PATCH bpf-next v2 9/9] xsk: document tx_metadata_len layout

[Stanislav Fomichev <sdf@google.com>]

- how to use
- how to query features
- pointers to the examples

Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 Documentation/networking/index.rst           |  1 +
 Documentation/networking/xsk-tx-metadata.rst | 77 ++++++++++++++++++++
 2 files changed, 78 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..b7289f06745c
--- /dev/null
+++ b/Documentation/networking/xsk-tx-metadata.rst
@@ -0,0 +1,77 @@
+==================
+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_TX_METADATA_TIMESTAMP``: requests the device to put transmission
+  timestamp into ``tx_timestamp`` field of ``union 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.
-- 
2.42.0.459.ge4e396fd5e-goog