[PATCH v14 06/18] net/pcap: rework transmit burst handling

Stephen Hemminger
2026-02-11T22:22:19Z

Replace the 64K stack-allocated bounce buffer with a per-queue
buffer allocated from hugepages via rte_malloc at queue setup.
This is necessary because the buffer may be used in a secondary
process transmit path where the primary process allocated it.

Fix error accounting: backpressure from pcap_sendpacket() (kernel
socket buffer full) was incorrectly counted as errors. Malformed
multi-segment mbufs where pkt_len exceeds actual data were silently
accepted; they are now detected via rte_pktmbuf_read() failure
and counted as errors.

Add datapath debug logging macros (PMD_RX_LOG, PMD_TX_LOG) gated
on RTE_ETHDEV_DEBUG_RX/TX. Advertise RTE_ETH_TX_OFFLOAD_MULTI_SEGS
in device capabilities since the driver has always supported
multi-segment transmit. Add tx_queue_release to free the bounce
buffer.

Fixes: fbbbf553f268 ("net/pcap: fix concurrent multiseg Tx")
Cc: stable@dpdk.org

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
drivers/net/pcap/pcap_ethdev.c | 122 +++++++++++++++++++++------------
drivers/net/pcap/pcap_osdep.h | 14 ++++
2 files changed, 92 insertions(+), 44 deletions(-)

diff --git a/drivers/net/pcap/pcap_ethdev.c b/drivers/net/pcap/pcap_ethdev.c
index 806451dc99..4761741edd 100644
--- a/drivers/net/pcap/pcap_ethdev.c
+++ b/drivers/net/pcap/pcap_ethdev.c
@@ -12,6 +12,7 @@
#include <errno.h>
#include <sys/time.h>
#include <sys/types.h>
+#include <unistd.h>
#include <pcap.h>

#include <rte_cycles.h>
@@ -91,6 +92,9 @@ struct pcap_tx_queue {
struct queue_stat tx_stat;
char name[PATH_MAX];
char type[ETH_PCAP_ARG_MAXLEN];
+
+ /* Temp buffer used for non-linear packets */
+ uint8_t *bounce_buf;
};

struct pmd_internals {
@@ -385,18 +389,17 @@ static uint16_t
eth_pcap_tx_dumper(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
{
unsigned int i;
- struct rte_mbuf *mbuf;
struct pmd_process_private *pp;
struct pcap_tx_queue *dumper_q = queue;
uint16_t num_tx = 0;
uint32_t tx_bytes = 0;
struct pcap_pkthdr header;
pcap_dumper_t *dumper;
- unsigned char temp_data[RTE_ETH_PCAP_SNAPLEN];
- size_t len, caplen;
+ unsigned char *temp_data;

pp = rte_eth_devices[dumper_q->port_id].process_private;
dumper = pp->tx_dumper[dumper_q->queue_id];
+ temp_data = dumper_q->bounce_buf;

if (dumper == NULL || nb_pkts == 0)
return 0;
@@ -404,25 +407,28 @@ eth_pcap_tx_dumper(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
/* writes the nb_pkts packets to the previously opened pcap file
* dumper */
for (i = 0; i < nb_pkts; i++) {
- mbuf = bufs[i];
+ struct rte_mbuf *mbuf = bufs[i];
+ uint32_t len, caplen;
+ const uint8_t *data;
+
len = caplen = rte_pktmbuf_pkt_len(mbuf);
- if (unlikely(!rte_pktmbuf_is_contiguous(mbuf) &&
- len > sizeof(temp_data))) {
- caplen = sizeof(temp_data);
- }

calculate_timestamp(&header.ts);
header.len = len;
header.caplen = caplen;
- /* rte_pktmbuf_read() returns a pointer to the data directly
- * in the mbuf (when the mbuf is contiguous) or, otherwise,
- * a pointer to temp_data after copying into it.
- */
- pcap_dump((u_char *)dumper, &header,
- rte_pktmbuf_read(mbuf, 0, caplen, temp_data));

- num_tx++;
- tx_bytes += caplen;
+ data = rte_pktmbuf_read(mbuf, 0, caplen, temp_data);
+ if (unlikely(data == NULL)) {
+ /* This only happens if mbuf is bogus pkt_len > data_len */
+ PMD_TX_LOG(ERR, "rte_pktmbuf_read failed");
+ dumper_q->tx_stat.err_pkts++;
+ } else {
+ pcap_dump((u_char *)dumper, &header, data);
+
+ num_tx++;
+ tx_bytes += caplen;
+ }
+
rte_pktmbuf_free(mbuf);
}

@@ -449,9 +455,6 @@ eth_tx_drop(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
uint32_t tx_bytes = 0;
struct pcap_tx_queue *tx_queue = queue;

- if (unlikely(nb_pkts == 0))
- return 0;
-
for (i = 0; i < nb_pkts; i++) {
tx_bytes += bufs[i]->pkt_len;
rte_pktmbuf_free(bufs[i]);
@@ -460,60 +463,74 @@ eth_tx_drop(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
tx_queue->tx_stat.pkts += nb_pkts;
tx_queue->tx_stat.bytes += tx_bytes;

- return i;
+ return nb_pkts;
}

/*
- * Callback to handle sending packets through a real NIC.
+ * Send a burst of packets to a pcap device.
+ *
+ * On Linux, pcap_sendpacket() calls send() on a blocking PF_PACKET
+ * socket with default kernel buffer sizes and no TX ring (PACKET_TX_RING).
+ * The send() call only blocks when the kernel socket send buffer is full,
+ * providing limited backpressure.
+ *
+ * On error, pcap_sendpacket() returns non-zero and the loop breaks,
+ * leaving remaining packets unsent.
+ *
+ * Bottom line: backpressure is not an error.
*/
static uint16_t
eth_pcap_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
{
unsigned int i;
- int ret;
- struct rte_mbuf *mbuf;
struct pmd_process_private *pp;
struct pcap_tx_queue *tx_queue = queue;
uint16_t num_tx = 0;
uint32_t tx_bytes = 0;
pcap_t *pcap;
- unsigned char temp_data[RTE_ETH_PCAP_SNAPLEN];
- size_t len;
+ unsigned char *temp_data;

pp = rte_eth_devices[tx_queue->port_id].process_private;
pcap = pp->tx_pcap[tx_queue->queue_id];
+ temp_data = tx_queue->bounce_buf;

if (unlikely(nb_pkts == 0 || pcap == NULL))
return 0;

for (i = 0; i < nb_pkts; i++) {
- mbuf = bufs[i];
- len = rte_pktmbuf_pkt_len(mbuf);
- if (unlikely(!rte_pktmbuf_is_contiguous(mbuf) &&
- len > sizeof(temp_data))) {
- PMD_LOG(ERR,
- "Dropping multi segment PCAP packet. Size (%zd) > max size (%zd).",
- len, sizeof(temp_data));
+ struct rte_mbuf *mbuf = bufs[i];
+ uint32_t len = rte_pktmbuf_pkt_len(mbuf);
+ const uint8_t *data;
+
+ if (unlikely(!rte_pktmbuf_is_contiguous(mbuf) && len > RTE_ETH_PCAP_SNAPSHOT_LEN)) {
+ PMD_TX_LOG(ERR,
+ "Dropping multi segment PCAP packet. Size (%u) > max size (%u).",
+ len, RTE_ETH_PCAP_SNAPSHOT_LEN);
+ tx_queue->tx_stat.err_pkts++;
rte_pktmbuf_free(mbuf);
continue;
}

- /* rte_pktmbuf_read() returns a pointer to the data directly
- * in the mbuf (when the mbuf is contiguous) or, otherwise,
- * a pointer to temp_data after copying into it.
- */
- ret = pcap_sendpacket(pcap,
- rte_pktmbuf_read(mbuf, 0, len, temp_data), len);
- if (unlikely(ret != 0))
- break;
- num_tx++;
- tx_bytes += len;
+ data = rte_pktmbuf_read(mbuf, 0, len, temp_data);
+ if (unlikely(data == NULL)) {
+ /* This only happens if mbuf is bogus pkt_len > data_len */
+ PMD_TX_LOG(ERR, "rte_pktmbuf_read failed");
+ tx_queue->tx_stat.err_pkts++;
+ } else {
+ /* Unfortunately, libpcap collapses transient (-EBUSY) and hard errors. */
+ if (pcap_sendpacket(pcap, data, len) != 0) {
+ PMD_TX_LOG(ERR, "pcap_sendpacket() failed: %s", pcap_geterr(pcap));
+ break;
+ }
+ num_tx++;
+ tx_bytes += len;
+ }
+
rte_pktmbuf_free(mbuf);
}

tx_queue->tx_stat.pkts += num_tx;
tx_queue->tx_stat.bytes += tx_bytes;
- tx_queue->tx_stat.err_pkts += i - num_tx;

return i;
}
@@ -753,6 +770,7 @@ eth_dev_info(struct rte_eth_dev *dev,
dev_info->max_rx_queues = dev->data->nb_rx_queues;
dev_info->max_tx_queues = dev->data->nb_tx_queues;
dev_info->min_rx_bufsize = 0;
+ dev_info->tx_offload_capa = RTE_ETH_TX_OFFLOAD_MULTI_SEGS;

return 0;
}
@@ -965,7 +983,7 @@ static int
eth_tx_queue_setup(struct rte_eth_dev *dev,
uint16_t tx_queue_id,
uint16_t nb_tx_desc __rte_unused,
- unsigned int socket_id __rte_unused,
+ unsigned int socket_id,
const struct rte_eth_txconf *tx_conf __rte_unused)
{
struct pmd_internals *internals = dev->data->dev_private;
@@ -973,11 +991,26 @@ eth_tx_queue_setup(struct rte_eth_dev *dev,

pcap_q->port_id = dev->data->port_id;
pcap_q->queue_id = tx_queue_id;
+ pcap_q->bounce_buf = rte_malloc_socket(NULL, RTE_ETH_PCAP_SNAPSHOT_LEN,
+ RTE_CACHE_LINE_SIZE, socket_id);
+ if (pcap_q->bounce_buf == NULL)
+ return -ENOMEM;
+
dev->data->tx_queues[tx_queue_id] = pcap_q;

return 0;
}

+static void
+eth_tx_queue_release(struct rte_eth_dev *dev, uint16_t tx_queue_id)
+{
+ struct pmd_internals *internals = dev->data->dev_private;
+ struct pcap_tx_queue *pcap_q = &internals->tx_queue[tx_queue_id];
+
+ rte_free(pcap_q->bounce_buf);
+ pcap_q->bounce_buf = NULL;
+}
+
static int
eth_rx_queue_start(struct rte_eth_dev *dev, uint16_t rx_queue_id)
{
@@ -1018,6 +1051,7 @@ static const struct eth_dev_ops ops = {
.dev_infos_get = eth_dev_info,
.rx_queue_setup = eth_rx_queue_setup,
.tx_queue_setup = eth_tx_queue_setup,
+ .tx_queue_release = eth_tx_queue_release,
.rx_queue_start = eth_rx_queue_start,
.tx_queue_start = eth_tx_queue_start,
.rx_queue_stop = eth_rx_queue_stop,
diff --git a/drivers/net/pcap/pcap_osdep.h b/drivers/net/pcap/pcap_osdep.h
index a0e2b5ace9..fe7399ff9f 100644
--- a/drivers/net/pcap/pcap_osdep.h
+++ b/drivers/net/pcap/pcap_osdep.h
@@ -13,6 +13,20 @@
extern int eth_pcap_logtype;
#define RTE_LOGTYPE_ETH_PCAP eth_pcap_logtype

+#ifdef RTE_ETHDEV_DEBUG_RX
+#define PMD_RX_LOG(level, ...) \
+ RTE_LOG_LINE_PREFIX(level, ETH_PCAP, "%s() rx: ", __func__, __VA_ARGS__)
+#else
+#define PMD_RX_LOG(...) do { } while (0)
+#endif
+
+#ifdef RTE_ETHDEV_DEBUG_TX
+#define PMD_TX_LOG(level, ...) \
+ RTE_LOG_LINE_PREFIX(level, ETH_PCAP, "%s() tx: ", __func__, __VA_ARGS__)
+#else
+#define PMD_TX_LOG(...) do { } while (0)
+#endif
+
int osdep_iface_index_get(const char *name);
int osdep_iface_mac_get(const char *name, struct rte_ether_addr *mac);

--
2.51.0