On Thu, 19 Oct 2023 at 19:50, Stanislav Fomichev <sdf@xxxxxxxxxx> wrote: > > - 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 | 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 > >
