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

[Stephen Hemminger <stephen@networkplumber.org>]

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