[PATCH v2] app/testpmd: add padding mode to txonly engine

[PATCH v2] app/testpmd: add padding mode to txonly engine

[Xingui Yang]

Add a new padding mode to the txonly forwarding engine, which allows
sending packets with configurable small sizes without standard L2/L3
headers. This is useful for testing NIC padding logic.

When padding mode is enabled via --tx-pkt-pad-mode flag:
- l2_len and l3_len are set to 0 instead of standard header lengths
- Packet data is filled with a static pattern instead of
  Ethernet/IP/UDP headers
- Minimum packet length validation is bypassed to allow small
  packet sizes (e.g., set txpkts 14)

Signed-off-by: Xingui Yang <yangxingui@huawei.com>
Signed-off-by: Huisong Li <lihuisong@huawei.com>
---
v2: Fix compilation exception of unterminated-string-initialization
---
 app/test-pmd/config.c     |  2 +-
 app/test-pmd/parameters.c |  7 +++++++
 app/test-pmd/testpmd.c    |  3 +++
 app/test-pmd/testpmd.h    |  1 +
 app/test-pmd/txonly.c     | 18 ++++++++++++++++--
 5 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/app/test-pmd/config.c b/app/test-pmd/config.c
index 9d457ca88e..36b9b023e2 100644
--- a/app/test-pmd/config.c
+++ b/app/test-pmd/config.c
@@ -6341,7 +6341,7 @@ set_tx_pkt_segments(unsigned int *seg_lengths, unsigned int nb_segs)
 		}
 		tx_pkt_len = (uint16_t)(tx_pkt_len + seg_lengths[i]);
 	}
-	if (tx_pkt_len < (sizeof(struct rte_ether_hdr) + 20 + 8)) {
+	if (tx_pkt_len < (sizeof(struct rte_ether_hdr) + 20 + 8) && !tx_pkt_pad_mode) {
 		fprintf(stderr, "total packet length=%u < %d - give up\n",
 				(unsigned) tx_pkt_len,
 				(int)(sizeof(struct rte_ether_hdr) + 20 + 8));
diff --git a/app/test-pmd/parameters.c b/app/test-pmd/parameters.c
index 337d8fc8ac..8c3b1244e7 100644
--- a/app/test-pmd/parameters.c
+++ b/app/test-pmd/parameters.c
@@ -195,6 +195,8 @@ enum {
 	TESTPMD_OPT_TXONLY_MULTI_FLOW_NUM,
 #define TESTPMD_OPT_TXONLY_FLOWS "txonly-flows"
 	TESTPMD_OPT_TXONLY_FLOWS_NUM,
+#define TESTPMD_OPT_TX_PKT_PAD_MODE "tx-pkt-pad-mode"
+	TESTPMD_OPT_TX_PKT_PAD_MODE_NUM,
 #define TESTPMD_OPT_RXQ_SHARE "rxq-share"
 	TESTPMD_OPT_RXQ_SHARE_NUM,
 #define TESTPMD_OPT_ETH_LINK_SPEED "eth-link-speed"
@@ -351,6 +353,7 @@ static const struct option long_options[] = {
 	NO_ARG(TESTPMD_OPT_MULTI_RX_MEMPOOL),
 	NO_ARG(TESTPMD_OPT_TXONLY_MULTI_FLOW),
 	REQUIRED_ARG(TESTPMD_OPT_TXONLY_FLOWS),
+	NO_ARG(TESTPMD_OPT_TX_PKT_PAD_MODE),
 	NO_ARG(TESTPMD_OPT_RXQ_SHARE),
 	REQUIRED_ARG(TESTPMD_OPT_ETH_LINK_SPEED),
 	NO_ARG(TESTPMD_OPT_DISABLE_LINK_CHECK),
@@ -504,6 +507,7 @@ usage(char* progname)
 	printf("  --txonly-multi-flow: generate multiple flows in txonly mode\n");
 	printf("  --txonly-nb-flows=N: number of flows per lcore in txonly"
 	       " multi-flow mode (1-64, default 64)\n");
+	printf("  --tx-pkt-pad-mode: enable padding mode in txonly mode\n");
 	printf("  --tx-ip=src,dst: IP addresses in Tx-only mode\n");
 	printf("  --tx-udp=src[,dst]: UDP ports in Tx-only mode\n");
 	printf("  --eth-link-speed: force link speed.\n");
@@ -1577,6 +1581,9 @@ launch_args_parse(int argc, char** argv)
 			else
 				rte_exit(EXIT_FAILURE, "txonly-flows must be >= 1 and <= 64\n");
 			break;
+		case TESTPMD_OPT_TX_PKT_PAD_MODE_NUM:
+			tx_pkt_pad_mode = 1;
+			break;
 		case TESTPMD_OPT_RXQ_SHARE_NUM:
 			rxq_share = 1;
 			break;
diff --git a/app/test-pmd/testpmd.c b/app/test-pmd/testpmd.c
index fcd8a90967..457bb6d3fe 100644
--- a/app/test-pmd/testpmd.c
+++ b/app/test-pmd/testpmd.c
@@ -296,6 +296,9 @@ uint32_t tx_pkt_times_inter;
 uint32_t tx_pkt_times_intra;
 /**< Timings for send scheduling in TXONLY mode, time between packets. */
 
+uint8_t tx_pkt_pad_mode;
+/**< Whether packet padding mode is enabled. */
+
 uint16_t nb_pkt_per_burst = DEF_PKT_BURST; /**< Number of packets per burst. */
 uint16_t nb_pkt_flowgen_clones; /**< Number of Tx packet clones to send in flowgen mode. */
 int nb_flows_flowgen = 1024; /**< Number of flows in flowgen mode. */
diff --git a/app/test-pmd/testpmd.h b/app/test-pmd/testpmd.h
index 3d4b36d668..04fdc2db42 100644
--- a/app/test-pmd/testpmd.h
+++ b/app/test-pmd/testpmd.h
@@ -663,6 +663,7 @@ extern uint16_t tx_pkt_seg_lengths[RTE_MAX_SEGS_PER_PKT]; /**< Seg. lengths */
 extern uint8_t  tx_pkt_nb_segs; /**< Number of segments in TX packets */
 extern uint32_t tx_pkt_times_intra;
 extern uint32_t tx_pkt_times_inter;
+extern uint8_t  tx_pkt_pad_mode;
 
 enum tx_pkt_split {
 	TX_PKT_SPLIT_OFF,
diff --git a/app/test-pmd/txonly.c b/app/test-pmd/txonly.c
index 64893fa205..2ddc100f21 100644
--- a/app/test-pmd/txonly.c
+++ b/app/test-pmd/txonly.c
@@ -192,8 +192,8 @@ pkt_burst_prepare(struct rte_mbuf *pkt, struct rte_mempool *mbp,
 	pkt->ol_flags |= ol_flags;
 	pkt->vlan_tci = vlan_tci;
 	pkt->vlan_tci_outer = vlan_tci_outer;
-	pkt->l2_len = sizeof(struct rte_ether_hdr);
-	pkt->l3_len = sizeof(struct rte_ipv4_hdr);
+	pkt->l2_len = tx_pkt_pad_mode ? 0 : sizeof(struct rte_ether_hdr);
+	pkt->l3_len = tx_pkt_pad_mode ? 0 : sizeof(struct rte_ipv4_hdr);
 
 	pkt_len = pkt->data_len;
 	pkt_seg = pkt;
@@ -204,6 +204,19 @@ pkt_burst_prepare(struct rte_mbuf *pkt, struct rte_mempool *mbp,
 		pkt_len += pkt_seg->data_len;
 	}
 	pkt_seg->next = NULL; /* Last segment of packet. */
+
+	if (tx_pkt_pad_mode) {
+		static const char pad_pattern[] = "0123456789abcdef";
+		uint32_t j;
+		char *pad;
+
+		pad = rte_pktmbuf_mtod(pkt, char *);
+		for (j = 0; j < pkt->data_len; j++)
+			pad[j] = pad_pattern[j % 16];
+
+		goto out;
+	}
+
 	/*
 	 * Copy headers in first packet segment(s).
 	 */
@@ -295,6 +308,7 @@ pkt_burst_prepare(struct rte_mbuf *pkt, struct rte_mempool *mbp,
 			sizeof(struct rte_ipv4_hdr) +
 			sizeof(pkt_udp_hdr));
 	}
+out:
 	/*
 	 * Complete first mbuf of packet and append it to the
 	 * burst of packets to be transmitted.
-- 
2.43.0

Re: [PATCH v2] app/testpmd: add padding mode to txonly engine

[Stephen Hemminger]

On Fri, 12 Jun 2026 17:12:17 +0800
Xingui Yang <yangxingui@huawei.com> wrote:

> Add a new padding mode to the txonly forwarding engine, which allows
> sending packets with configurable small sizes without standard L2/L3
> headers. This is useful for testing NIC padding logic.
> 
> When padding mode is enabled via --tx-pkt-pad-mode flag:
> - l2_len and l3_len are set to 0 instead of standard header lengths
> - Packet data is filled with a static pattern instead of
>   Ethernet/IP/UDP headers
> - Minimum packet length validation is bypassed to allow small
>   packet sizes (e.g., set txpkts 14)
> 
> Signed-off-by: Xingui Yang <yangxingui@huawei.com>
> Signed-off-by: Huisong Li <lihuisong@huawei.com>
> ---
> v2: Fix compilation exception of unterminated-string-initialization
> ---

What about something like this (*not tested*) patch.

From 84ff35849f9881a93eed65ccd43a5cd1197cecb8 Mon Sep 17 00:00:00 2001
From: Stephen Hemminger <stephen@networkplumber.org>
Date: Fri, 12 Jun 2026 09:10:17 -0700
Subject: [PATCH] testpnd: allow configuring runt frames

Allow setting transmit size to be a small value which has
ethernet header but no IP or UDP header.
---
 app/test-pmd/config.c                       | 12 +++----
 app/test-pmd/txonly.c                       | 35 +++++++++++++++++++--
 doc/guides/testpmd_app_ug/testpmd_funcs.rst | 13 ++++++++
 3 files changed, 51 insertions(+), 9 deletions(-)

diff --git a/app/test-pmd/config.c b/app/test-pmd/config.c
index 9d457ca88e..46ff678b9f 100644
--- a/app/test-pmd/config.c
+++ b/app/test-pmd/config.c
@@ -6327,9 +6327,6 @@ set_tx_pkt_segments(unsigned int *seg_lengths, unsigned int nb_segs)
 	/*
 	 * Check that each segment length is greater or equal than
 	 * the mbuf data size.
-	 * Check also that the total packet length is greater or equal than the
-	 * size of an empty UDP/IP packet (sizeof(struct rte_ether_hdr) +
-	 * 20 + 8).
 	 */
 	tx_pkt_len = 0;
 	for (i = 0; i < nb_segs; i++) {
@@ -6341,10 +6338,11 @@ set_tx_pkt_segments(unsigned int *seg_lengths, unsigned int nb_segs)
 		}
 		tx_pkt_len = (uint16_t)(tx_pkt_len + seg_lengths[i]);
 	}
-	if (tx_pkt_len < (sizeof(struct rte_ether_hdr) + 20 + 8)) {
-		fprintf(stderr, "total packet length=%u < %d - give up\n",
-				(unsigned) tx_pkt_len,
-				(int)(sizeof(struct rte_ether_hdr) + 20 + 8));
+
+	/* Allow runt packets this is a test tool. */
+	if (tx_pkt_len < (sizeof(struct rte_ether_hdr))) {
+		fprintf(stderr, "total packet length=%u < %zu - give up\n",
+			tx_pkt_len, sizeof(struct rte_ether_hdr));
 		return;
 	}
 
diff --git a/app/test-pmd/txonly.c b/app/test-pmd/txonly.c
index 64893fa205..2e0abe361e 100644
--- a/app/test-pmd/txonly.c
+++ b/app/test-pmd/txonly.c
@@ -76,6 +76,12 @@ copy_buf_to_pkt_segs(void* buf, unsigned len, struct rte_mbuf *pkt,
 	while (offset >= seg->data_len) {
 		offset -= seg->data_len;
 		seg = seg->next;
+		/*
+		 * The packet may be shorter than the header stack when
+		 * generating runt frames; stop once it runs out of segments.
+		 */
+		if (seg == NULL)
+			return;
 	}
 	copy_len = seg->data_len - offset;
 	seg_buf = rte_pktmbuf_mtod_offset(seg, char *, offset);
@@ -84,6 +90,8 @@ copy_buf_to_pkt_segs(void* buf, unsigned len, struct rte_mbuf *pkt,
 		len -= copy_len;
 		buf = ((char*) buf + copy_len);
 		seg = seg->next;
+		if (seg == NULL)
+			return;
 		seg_buf = rte_pktmbuf_mtod(seg, char *);
 		copy_len = seg->data_len;
 	}
@@ -193,7 +201,6 @@ pkt_burst_prepare(struct rte_mbuf *pkt, struct rte_mempool *mbp,
 	pkt->vlan_tci = vlan_tci;
 	pkt->vlan_tci_outer = vlan_tci_outer;
 	pkt->l2_len = sizeof(struct rte_ether_hdr);
-	pkt->l3_len = sizeof(struct rte_ipv4_hdr);
 
 	pkt_len = pkt->data_len;
 	pkt_seg = pkt;
@@ -204,6 +211,24 @@ pkt_burst_prepare(struct rte_mbuf *pkt, struct rte_mempool *mbp,
 		pkt_len += pkt_seg->data_len;
 	}
 	pkt_seg->next = NULL; /* Last segment of packet. */
+
+	/*
+	 * A runt frame may be too short to carry a full IPv4/UDP header.
+	 * Clamp l3_len and drop any checksum offload whose header is not
+	 * fully present, so the PMD is never asked to checksum bytes that
+	 * are not in the frame. pkt_len is at least sizeof(rte_ether_hdr),
+	 * so the subtraction below cannot underflow.
+	 */
+	pkt->l3_len = RTE_MIN(sizeof(struct rte_ipv4_hdr),
+			      pkt_len - sizeof(struct rte_ether_hdr));
+	if (pkt_len < sizeof(struct rte_ether_hdr) +
+			sizeof(struct rte_ipv4_hdr))
+		pkt->ol_flags &= ~(RTE_MBUF_F_TX_IP_CKSUM |
+				   RTE_MBUF_F_TX_L4_MASK);
+	else if (pkt_len < sizeof(struct rte_ether_hdr) +
+			sizeof(struct rte_ipv4_hdr) +
+			sizeof(struct rte_udp_hdr))
+		pkt->ol_flags &= ~RTE_MBUF_F_TX_L4_MASK;
 	/*
 	 * Copy headers in first packet segment(s).
 	 */
@@ -405,7 +430,13 @@ tx_only_begin(portid_t pi)
 	pkt_hdr_len = (uint16_t)(sizeof(struct rte_ether_hdr) +
 				 sizeof(struct rte_ipv4_hdr) +
 				 sizeof(struct rte_udp_hdr));
-	pkt_data_len = tx_pkt_length - pkt_hdr_len;
+	/*
+	 * tx_pkt_length may be smaller than the full header stack when
+	 * generating runt frames; clamp the payload length to zero in that
+	 * case so the IP/UDP length fields stay sane.
+	 */
+	pkt_data_len = tx_pkt_length > pkt_hdr_len ?
+			tx_pkt_length - pkt_hdr_len : 0;
 
 	if ((tx_pkt_split == TX_PKT_SPLIT_RND || txonly_multi_flow) &&
 	    tx_pkt_seg_lengths[0] < pkt_hdr_len) {
diff --git a/doc/guides/testpmd_app_ug/testpmd_funcs.rst b/doc/guides/testpmd_app_ug/testpmd_funcs.rst
index f0f2b0758b..d2e5b63586 100644
--- a/doc/guides/testpmd_app_ug/testpmd_funcs.rst
+++ b/doc/guides/testpmd_app_ug/testpmd_funcs.rst
@@ -876,6 +876,19 @@ Set the length of each segment of the TX-ONLY packets or length of packet for FL
 
 Where x[,y]* represents a CSV list of values, without white space.
 
+The total packet length may be set as small as the Ethernet header
+(``sizeof(struct rte_ether_hdr)``), which is below the size of an empty
+UDP/IPv4 packet. This generates runt frames with a truncated (or absent)
+IPv4/UDP header, which is useful for testing how a driver handles
+undersized frames. Any header bytes that do not fit in the requested
+length are simply not emitted. Checksum offloads are automatically
+dropped for a frame too short to contain the corresponding header.
+
+Note that random split (``set txsplit rand``) and multi-flow
+(``set txonly-flows``) still require the first segment to hold the full
+Ethernet/IPv4/UDP header stack, so they cannot be combined with runt
+lengths.
+
 set txtimes
 ~~~~~~~~~~~
 
-- 
2.53.0

Re: [PATCH v2] app/testpmd: add padding mode to txonly engine

[yangxingui]

On 2026/6/13 0:13, Stephen Hemminger wrote:
> On Fri, 12 Jun 2026 17:12:17 +0800
> Xingui Yang <yangxingui@huawei.com> wrote:
> 
>> Add a new padding mode to the txonly forwarding engine, which allows
>> sending packets with configurable small sizes without standard L2/L3
>> headers. This is useful for testing NIC padding logic.
>>
>> When padding mode is enabled via --tx-pkt-pad-mode flag:
>> - l2_len and l3_len are set to 0 instead of standard header lengths
>> - Packet data is filled with a static pattern instead of
>>    Ethernet/IP/UDP headers
>> - Minimum packet length validation is bypassed to allow small
>>    packet sizes (e.g., set txpkts 14)
>>
>> Signed-off-by: Xingui Yang <yangxingui@huawei.com>
>> Signed-off-by: Huisong Li <lihuisong@huawei.com>
>> ---
>> v2: Fix compilation exception of unterminated-string-initialization
>> ---
> 
> What about something like this (*not tested*) patch.

Hi Stephen,

Thank you for the valuable suggestion! Your approach is cleaner and
more elegant for generating runt frames with truncated headers.

However, our use case requires sending packets smaller than the
Ethernet header size (e.g., `set txpkts 9` or `set txpkts 2`).
These ultra-small packets are needed to test NIC padding logic where
no standard L2/L3 headers are present at all.

We could combine both approaches:
- Allow packet lengths down to 1 byte (remove the 14-byte minimum)
- Dynamically compute l2_len/l3_len based on actual packet length
- Use a fill pattern for packets too small to hold Ethernet header
- Keep the checksum offload handling from your patch

This way testpmd can support the full range:
- Normal packets (>= 14 + 20 + 8 bytes): full headers with checksum
- Runt frames (14-42 bytes): truncated headers, checksums disabled
- Ultra-small packets (<14 bytes): fill pattern only, l2_len/l3_len = 0

Would this combined approach be acceptable? We can submit a v3 patch
incorporating your suggestions plus support for ultra-small packets.

Thanks,
Xingui Yang