[dpdk-users] mlx5: packets lost between good+discard and phy counters

[Gerry Wan]

I have a simple forwarding experiment using a mlx5 NIC directly connected
to a generator. I am noticing that at high enough throughput,
rx_good_packets + rx_phy_discard_packets may not equal rx_phy_packets.
Where are these packets being dropped?

Below is an example xstats where I receive at almost the limit of what my
application can handle with no loss. It shows rx_phy_discard_packets is 0
but the number actually received by the CPU is less than rx_phy_packets.
rx_out_of_buffer and other errors are also 0.

I have disabled Ethernet flow control via rte_eth_dev_flow_ctrl_set with
mode = RTE_FC_NONE, if that matters.

{
    "rx_good_packets": 319992439,
    "tx_good_packets": 0,
    "rx_good_bytes": 19199546340,
    "tx_good_bytes": 0,
    "rx_missed_errors": 0,
    "rx_errors": 0,
    "tx_errors": 0,
    "rx_mbuf_allocation_errors": 0,
    "rx_q0_packets": 319992439,
    "rx_q0_bytes": 19199546340,
    "rx_q0_errors": 0,
    "rx_wqe_errors": 0,
    "rx_unicast_packets": 319999892,
    "rx_unicast_bytes": 19199993520,
    "tx_unicast_packets": 0,
    "tx_unicast_bytes": 0,
    "rx_multicast_packets": 0,
    "rx_multicast_bytes": 0,
    "tx_multicast_packets": 0,
    "tx_multicast_bytes": 0,
    "rx_broadcast_packets": 0,
    "rx_broadcast_bytes": 0,
    "tx_broadcast_packets": 0,
    "tx_broadcast_bytes": 0,
    "tx_phy_packets": 0,
    "rx_phy_packets": 319999892,
    "rx_phy_crc_errors": 0,
    "tx_phy_bytes": 0,
    "rx_phy_bytes": 20479993088,
    "rx_phy_in_range_len_errors": 0,
    "rx_phy_symbol_errors": 0,
    "rx_phy_discard_packets": 0,
    "tx_phy_discard_packets": 0,
    "tx_phy_errors": 0,
    "rx_out_of_buffer": 0,
    "tx_pp_missed_interrupt_errors": 0,
    "tx_pp_rearm_queue_errors": 0,
    "tx_pp_clock_queue_errors": 0,
    "tx_pp_timestamp_past_errors": 0,
    "tx_pp_timestamp_future_errors": 0,
    "tx_pp_jitter": 0,
    "tx_pp_wander": 0,
    "tx_pp_sync_lost": 0,
}