> [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. Thanks for the review. zxdh_xmit_pkts_simple is installed only when the application does not request RTE_ETH_TX_OFFLOAD_MULTI_SEGS. It is the fast path for single-segment traffic; zxdh_xmit_pkts_packed is the general path that handles multi-segment, full per-packet validation, and arbitrary headroom. The simple path's precondition is data_off >= hdr_len. static_assert(RTE_PKTMBUF_HEADROOM >= ZXDH_DL_NET_HDR_SIZE) ensures default headroom fits the header, so freshly allocated mbufs from a conformant mempool satisfy it. The runtime break is a defensive guard, not the error path. A short return signals "this batch is not for this path",not ring backpressure. The caller should keep MULTI_SEGS enabled (so packed is selected) or use the tx_pkt_prepare hook (zxdh_xmit_pkts_simple_prepare), which already validates data_off per packet and reports the offending index. Adding free-and-compact into the simple path would reintroduce a per-packet branch and an in-place compaction loop on every burst — the overhead the fast path exists to avoid. To make this explicit: zxdh_xmit_pkts_simple is restricted to single-segment mbufs with data_off >= hdr_len; anything outside the contract — including reduced-headroom mbufs — can go through zxdh_xmit_pkts_packed.