> [PATCH v8 4/4] net/zxdh: optimize Tx xmit pkts performance > > Error: the simple Tx burst signals a bad packet with a short return, > which the application cannot distinguish from backpressure. > for (i = 0; i < nb_pkts; i++) { > rte_prefetch0(tx_pkts[i]); > if (unlikely(tx_pkts[i]->data_off < hdr_len)) { > txvq->stats.errors += nb_pkts - i; > nb_pkts = i; > break; > } > } > > A short return from tx_burst is the backpressure signal (transmit ring > full, retry later). Here it is also used to mean "packet i is bad", > and the bad mbuf is left owned by the caller. The application has no > way to tell the two apart: the usual > > for (sent = 0; sent < n; ) > sent += rte_eth_tx_burst(port, q, &pkts[sent], n - sent); > > loop treats the short return as backpressure and resubmits pkts[i], > which fails again every time -- head-of-line blocking, and the good > packets after i (which had ring space) never go out. > > A packet that cannot be sent must be consumed by the driver, not > handed back. Free it in tx_burst, increment the tx error counter, and > continue with the rest of the burst. For a burst of 16 where only > index 3 is bad and the ring has room, tx_burst should return 16, with > stats showing 15 transmitted and 1 tx error. A short return is then > reserved for the one case the application is entitled to retry: ring > full. The design intent of zxdh_xmit_pkts_simple is a fast path with a usage restriction, not a general Tx entry point. The restriction is data_off >= hdr_len on single-segment mbufs. When a packet violates the restriction, the function exits without sending — that is the contract, not a bug. Applications that may produce reduced-headroom mbufs are expected to keep RTE_ETH_TX_OFFLOAD_MULTI_SEGS enabled, in which case zxdh_xmit_pkts_packed is selected instead and handles the full set of inputs. The split between the two paths is intentional. how about add a paragraph to the "Limitations or Known issues" section of doc/guides/nics/zxdh.rst describing the fast path's requirement and pointing at the packed path as the alternative. Thanks.