AF_XDP TX Metadata

This document describes how to enable offloads when transmitting packets via AF_XDP. Refer to 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.

  • XDP_TXMD_FLAGS_LAUNCH_TIME: requests the device to schedule the packet for transmission at a pre-determined time called launch time. The value of launch time is indicated by launch_time field of union xsk_tx_metadata.

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.

Launch Time

The value of the requested launch time should be based on the device’s PTP Hardware Clock (PHC) to ensure accuracy. AF_XDP takes a different data path compared to the ETF queuing discipline, which organizes packets and delays their transmission. Instead, AF_XDP immediately hands off the packets to the device driver without rearranging their order or holding them prior to transmission. Since the driver maintains FIFO behavior and does not perform packet reordering, a packet with a launch time request will block other packets in the same Tx Queue until it is sent. Therefore, it is recommended to allocate separate queue for scheduling traffic that is intended for future transmission.

In scenarios where the launch time offload feature is disabled, the device driver is expected to disregard the launch time request. For correct interpretation and meaningful operation, the launch time should never be set to a value larger than the farthest programmable time in the future (the horizon). Different devices have different hardware limitations on the launch time offload feature.

stmmac driver

For stmmac, TSO and launch time (TBS) features are mutually exclusive for each individual Tx Queue. By default, the driver configures Tx Queue 0 to support TSO and the rest of the Tx Queues to support TBS. The launch time hardware offload feature can be enabled or disabled by using the tc-etf command to call the driver’s ndo_setup_tc() callback.

The value of the launch time that is programmed in the Enhanced Normal Transmit Descriptors is a 32-bit value, where the most significant 8 bits represent the time in seconds and the remaining 24 bits represent the time in 256 ns increments. The programmed launch time is compared against the PTP time (bits[39:8]) and rolls over after 256 seconds. Therefore, the horizon of the launch time for dwmac4 and dwxlgmac2 is 128 seconds in the future.

igc driver

For igc, all four Tx Queues support the launch time feature. The launch time hardware offload feature can be enabled or disabled by using the tc-etf command to call the driver’s ndo_setup_tc() callback. When entering TSN mode, the igc driver will reset the device and create a default Qbv schedule with a 1-second cycle time, with all Tx Queues open at all times.

The value of the launch time that is programmed in the Advanced Transmit Context Descriptor is a relative offset to the starting time of the Qbv transmission window of the queue. The Frst flag of the descriptor can be set to schedule the packet for the next Qbv cycle. Therefore, the horizon of the launch time for i225 and i226 is the ending time of the next cycle of the Qbv transmission window of the queue. For example, when the Qbv cycle time is set to 1 second, the horizon of the launch time ranges from 1 second to 2 seconds, depending on where the Qbv cycle is currently running.

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

  • tx-launch-time-fifo: device supports XDP_TXMD_FLAGS_LAUNCH_TIME

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.