From 9620e956d5b592d305728144931e89d780a598aa Mon Sep 17 00:00:00 2001 From: Stanislav Fomichev Date: Mon, 27 Nov 2023 11:03:12 -0800 Subject: xsk: Document tx_metadata_len layout - how to use - how to query features - pointers to the examples Signed-off-by: Stanislav Fomichev Link: https://lore.kernel.org/r/20231127190319.1190813-7-sdf@google.com Signed-off-by: Alexei Starovoitov --- Documentation/networking/index.rst | 1 + Documentation/networking/xdp-rx-metadata.rst | 2 + Documentation/networking/xsk-tx-metadata.rst | 70 ++++++++++++++++++++++++++++ 3 files changed, 73 insertions(+) create mode 100644 Documentation/networking/xsk-tx-metadata.rst diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst index 683eb42309cc..a297a894b366 100644 --- a/Documentation/networking/index.rst +++ b/Documentation/networking/index.rst @@ -123,6 +123,7 @@ Contents: xfrm_sync xfrm_sysctl xdp-rx-metadata + xsk-tx-metadata .. only:: subproject and html diff --git a/Documentation/networking/xdp-rx-metadata.rst b/Documentation/networking/xdp-rx-metadata.rst index 205696780b78..e3e9420fd817 100644 --- a/Documentation/networking/xdp-rx-metadata.rst +++ b/Documentation/networking/xdp-rx-metadata.rst @@ -1,3 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + =============== XDP RX Metadata =============== diff --git a/Documentation/networking/xsk-tx-metadata.rst b/Documentation/networking/xsk-tx-metadata.rst new file mode 100644 index 000000000000..4f376560b23f --- /dev/null +++ b/Documentation/networking/xsk-tx-metadata.rst @@ -0,0 +1,70 @@ +================== +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. + +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. -- cgit v1.2.3