mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-12-28 16:56:26 +00:00
d5e726d914
Julian reports that commit341ac980ea("xsk: Support tx_metadata_len") can break existing use cases which don't zero-initialize xdp_umem_reg padding. Introduce new XDP_UMEM_TX_METADATA_LEN to make sure we interpret the padding as tx_metadata_len only when being explicitly asked. Fixes:341ac980ea("xsk: Support tx_metadata_len") Reported-by: Julian Schindel <mail@arctic-alpaca.de> Signed-off-by: Stanislav Fomichev <sdf@fomichev.me> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com> Link: https://lore.kernel.org/bpf/20240713015253.121248-2-sdf@fomichev.me
86 lines
3.4 KiB
ReStructuredText
86 lines
3.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
==================
|
|
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`` and
|
|
``XDP_UMEM_TX_METADATA_LEN`` flag 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)``.
|
|
|
|
Note that in the original implementation the ``XDP_UMEM_TX_METADATA_LEN``
|
|
flag was not required. Applications might attempt to create a umem
|
|
with a flag first and if it fails, do another attempt without a flag.
|
|
|
|
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.
|
|
|
|
Software TX Checksum
|
|
====================
|
|
|
|
For development and testing purposes its possible to pass
|
|
``XDP_UMEM_TX_SW_CSUM`` flag to ``XDP_UMEM_REG`` UMEM registration call.
|
|
In this case, when running in ``XDK_COPY`` mode, the TX checksum
|
|
is calculated on the CPU. Do not enable this option in production because
|
|
it will negatively affect performance.
|
|
|
|
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.
|