netdev-dpdk: Dump packets that fail Tx preparation.

It's hard to debug situations where driver rejects packets for some
reason.  Dumping out the mbuf should help with that.

Sample output looks like this:

  |netdev_dpdk(pmd-c03/id:8)|DBG|ovs-p1: First invalid packet:
  dump mbuf at 0x1180bce140, iova=0x2cb7ce400, buf_len=2176
    pkt_len=64, ol_flags=0x2, nb_segs=1, port=65535, ptype=0
    segment at 0x1180bce140, data=0x1180bce580, len=90, off=384, refcnt=1
    Dump data at [0x1180bce580], len=64
  00000000: 33 33 00 00 00 16 AA 27 91 F9 4D 96 86 DD 60 00 | 33.....'..M...`.
  00000010: 00 00 00 24 00 01 00 00 00 00 00 00 00 00 00 00 | ...$............
  00000020: 00 00 00 00 00 00 FF 02 00 00 00 00 00 00 00 00 | ................
  00000030: 00 00 00 00 00 16 3A 00 05 02 00 00 01 00 8F 00 | ......:.........

Acked-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/netdev-dpdk.c

@@ -2664,6 +2664,35 @@ netdev_dpdk_prep_hwol_batch(struct netdev_dpdk *dev, struct rte_mbuf **pkts,
     return cnt;
 }
 
+static void
+netdev_dpdk_mbuf_dump(const char *prefix, const char *message,
+                      const struct rte_mbuf *mbuf)
+{
+    static struct vlog_rate_limit dump_rl = VLOG_RATE_LIMIT_INIT(5, 5);
+    char *response = NULL;
+    FILE *stream;
+    size_t size;
+
+    if (VLOG_DROP_DBG(&dump_rl)) {
+        return;
+    }
+
+    stream = open_memstream(&response, &size);
+    if (!stream) {
+        VLOG_ERR("Unable to open memstream for mbuf dump: %s.",
+                 ovs_strerror(errno));
+        return;
+    }
+
+    rte_pktmbuf_dump(stream, mbuf, rte_pktmbuf_pkt_len(mbuf));
+
+    fclose(stream);
+
+    VLOG_DBG(prefix ? "%s: %s:\n%s" : "%s%s:\n%s",
+             prefix ? prefix : "", message, response);
+    free(response);
+}
+
 /* Tries to transmit 'pkts' to txq 'qid' of device 'dev'.  Takes ownership of
  * 'pkts', even in case of failure.
  *
@@ -2680,6 +2709,8 @@ netdev_dpdk_eth_tx_burst(struct netdev_dpdk *dev, int qid,
         VLOG_WARN_RL(&rl, "%s: Output batch contains invalid packets. "
                      "Only %u/%u are valid: %s", netdev_get_name(&dev->up),
                      nb_tx_prep, cnt, rte_strerror(rte_errno));
+        netdev_dpdk_mbuf_dump(netdev_get_name(&dev->up),
+                              "First invalid packet", pkts[nb_tx_prep]);
     }
 
     while (nb_tx != nb_tx_prep) {