126. Scattered Packets Tests

The support of scattered packets by Poll Mode Drivers consists in making it possible to receive and to transmit scattered multi-segments packets composed of multiple non-contiguous memory buffers. To enforce the receipt of scattered packets, the DMA rings of port RX queues must be configured with mbuf data buffers whose size is lower than the maximum frame length. The forwarding of scattered input packets naturally enforces the transmission of scattered packets by PMD transmit functions.

126.1. Configuring the size of mbuf data buffers

The size of mbuf data buffers is configured with the parameter --mbuf-size that is supplied in the set of parameters when launching the testpmd application. The default size of the mbuf data buffer is 2048 so that a full 1518-byte (CRC included) Ethernet frame can be stored in a mono-segment packet.

126.2. Functional Tests of Scattered Packets

Testing the support of scattered packets in Poll Mode Drivers consists in sending to the test machine packets whose length is greater than the size of mbuf data buffers used to populate the DMA rings of port RX queues.

First, the receipt and the transmission of scattered packets must be tested with the CRC stripping option enabled, which guarantees that scattered packets only contain packet data.

In addition, the support of scattered packets must also be performed with the CRC stripping option disabled, to check the special cases of scattered input packets whose last buffer only contains the whole CRC or part of it. In such cases, PMD receive functions must free the last buffer when removing the CRC from the packet before returning it.

As a whole, the following packet lengths (CRC included) must be tested to check all packet memory configurations:

  1. packet length < mbuf data buffer size
  2. packet length = mbuf data buffer size
  3. packet length = mbuf data buffer size + 1
  4. packet length = mbuf data buffer size + 4
  5. packet length = mbuf data buffer size + 5

In cases 1) and 2), the hardware RX engine stores the packet data and the CRC in a single buffer.

In case 3), the hardware RX engine stores the packet data and the 3 first bytes of the CRC in the first buffer, and the last byte of the CRC in a second buffer.

In case 4), the hardware RX engine stores all the packet data in the first buffer, and the CRC in a second buffer.

In case 5), the hardware RX engine stores part of the packet data in the first buffer, and the last data byte plus the CRC in a second buffer.

126.3. Prerequisites

Assuming that ports 0 and 1 of the test target are directly connected to a Traffic Generator, launch the testpmd application with the following arguments:

./x86_64-native-linuxapp-gcc/app/testpmd -c 0x6 -n 4 -- -i --mbcache=200 \
--mbuf-size=2048 --portmask=0x1 --max-pkt-len=9000 --port-topology=loop \
--tx-offloads=DEV_TX_OFFLOAD_MULTI_SEGS

The -n command is used to select the number of memory channels. It should match the number of memory channels on that setup.

DEV_TX_OFFLOAD_MULTI_SEGS is a TX offload capability, means device supports multi segment send. Defined in DPDK code lib/librte_ethdev/rte_ethdev.h:

#define DEV_TX_OFFLOAD_MULTI_SEGS       0x00008000

126.4. Test Case: Scatter Mbuf 2048

Start packet forwarding in the testpmd application with the start command. Send 5 packets,the lengths are mbuf-size + offset (CRC included). The offset are -1, 0, 1, 4, 5 respectively. Check that the same amount of frames and bytes are received back by the Traffic Generator from it’s port connected to the target’s port 1.