[xdp-hints] Re: [PATCH bpf-next v4 11/11] xsk: Document tx_metadata_len layout

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Thu, 19 Oct 2023 at 19:50, Stanislav Fomichev <sdf@google.com> wrote:
>
> - 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 2ffc5ad10295..f3c2566d6cad 100644
> --- a/Documentation/networking/index.rst
> +++ b/Documentation/networking/index.rst
> @@ -122,6 +122,7 @@ 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

nit: of there -> where

> +  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

nit: not -> note

> +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.
> --
> 2.42.0.655.g421f12c284-goog
>
>