Re: [PATCH v6 4/4] tcp_vu: Support multibuffer frames in tcp_vu_send_flag()

20 May 2026

On 5/11/26 12:52, David Gibson wrote:
...
On Thu, Apr 16, 2026 at 06:16:18PM +0200, Laurent Vivier wrote:
...
Build the Ethernet, IP, and TCP headers on the stack instead of
directly in the buffer via pointer casts, then write them into the
iovec with IOV_PUSH_HEADER().  This mirrors the approach already used
in tcp_vu_prepare() and udp_vu_prepare().
Remove the vu_eth(), vu_ip(), vu_payloadv4() and vu_payloadv6() helpers
from vu_common.h, as they are no longer used anywhere.
Introduce tcp_vu_send_dup() to handle DUP_ACK duplication using
vu_collect() and iov_memcopy() instead of a plain memcpy(), so that
s/iov_memcopy/iov_memcpy/
...
the duplicated frame is also properly scattered across multiple iovecs.
Signed-off-by: Laurent Vivier 
---
  iov.c       |   1 -
  tcp_vu.c    | 153 ++++++++++++++++++++++++++++++----------------------
  vu_common.h |  20 -------
  3 files changed, 90 insertions(+), 84 deletions(-)

diff --git a/iov.c b/iov.c
index c0d9c6d21322..31fc3e479572 100644
--- a/iov.c
+++ b/iov.c
@@ -208,7 +208,6 @@ void iov_memset(const struct iovec *iov, size_t iov_cnt, size_t offset, int c,
   *
   * Return: total number of bytes copied
   */
-/* cppcheck-suppress unusedFunction */
  size_t iov_memcpy(struct iovec *dst_iov, size_t dst_iov_cnt, size_t dst_offset,
        const struct iovec *src_iov, size_t src_iov_cnt,
        size_t src_offset, size_t length)
diff --git a/tcp_vu.c b/tcp_vu.c
index 96b16007701d..7f7e43860b10 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -74,6 +74,42 @@ static size_t tcp_vu_hdrlen(bool v6)
  return hdrlen;
  }
+/**
+ * tcp_vu_send_dup() - Duplicate a frame into a new virtqueue element
+ * @c:		Execution context
+ * @vq:		Receive virtqueue
+ * @dest_elem:	Destination virtqueue element to collect
+ * @dest_iov:	Destination iovec array for collected buffers
+ * @max_dest_iov: Maximum number of entries in @dest_iov
+ * @src_iov:	Source iovec array containing the frame to duplicate
+ * @src_cnt:	Number of entries in @src_iov
As Jon noted, @vnlen is missing.
...
+ *
+ * Return: number of virtqueue elements collected (0 if none available)
+ */
+static int tcp_vu_send_dup(const struct ctx *c, struct vu_virtq *vq,
+			   struct vu_virtq_element *dest_elem,
+			   struct iovec *dest_iov, size_t max_dest_iov,
+			   const struct iovec *src_iov, size_t src_cnt,
+			   size_t vnlen)
+{
+	const struct vu_dev *vdev = c->vdev;
+	size_t dest_cnt;
+	int elem_cnt;
+
+	elem_cnt = vu_collect(vdev, vq, dest_elem, 1, dest_iov, max_dest_iov,
+			      &dest_cnt, vnlen, NULL);
+	if (elem_cnt == 0)
+		return 0;
+
+	iov_memcpy(dest_iov, dest_cnt, 0, src_iov, src_cnt, 0,
+		   MAX(VNET_HLEN + ETH_ZLEN, vnlen));
+
+	if (*c->pcap)
+		pcap_iov(dest_iov, dest_cnt, VNET_HLEN, vnlen - VNET_HLEN);
+
+	return elem_cnt;
+}
+
  /**
   * tcp_vu_send_flag() - Send segment with flags to vhost-user (no payload)
   * @c:		Execution context
@@ -86,97 +122,88 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
  {
  struct vu_dev *vdev = c->vdev;
  struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
+	size_t optlen, hdrlen, iov_cnt, iov_used;
  struct vu_virtq_element flags_elem[2];
Does the rationale for this having only 2 elements still apply?
Yes, this is element, not iovec. One for the flags, an other one for the DUP.
...
...
-	size_t optlen, hdrlen, l2len;
-	struct ipv6hdr *ip6h = NULL;
-	struct iphdr *ip4h = NULL;
-	struct iovec flags_iov[2];
-	struct tcp_syn_opts *opts;
+	struct iovec flags_iov[64];
+	int elem_cnt, dup_elem_cnt = 0;
+	struct tcp_syn_opts opts;
  struct iov_tail payload;
-	struct tcphdr *th;
-	struct ethhdr *eh;
+	struct ipv6hdr ip6h;
+	struct iphdr ip4h;
+	struct tcphdr th;
+	struct ethhdr eh;
  uint32_t seq;
-	int elem_cnt;
  int ret;
hdrlen = tcp_vu_hdrlen(CONN_V6(conn));
elem_cnt = vu_collect(vdev, vq, &flags_elem[0], 1,
-			      &flags_iov[0], 1, NULL,
-			      hdrlen + sizeof(*opts), NULL);
-	if (elem_cnt != 1)
+			      flags_iov, ARRAY_SIZE(flags_iov), &iov_cnt,
+			      hdrlen + sizeof(opts), NULL);
+	if (elem_cnt == 0)
      return -1;
-	assert(flags_elem[0].in_num == 1);
-	assert(flags_elem[0].in_sg[0].iov_len >=
-	       MAX(hdrlen + sizeof(*opts), ETH_ZLEN + VNET_HLEN));
-
-	eh = vu_eth(flags_elem[0].in_sg[0].iov_base);
-
-	memcpy(eh->h_dest, c->guest_mac, sizeof(eh->h_dest));
-	memcpy(eh->h_source, c->our_tap_mac, sizeof(eh->h_source));
-
-	if (CONN_V4(conn)) {
-		eh->h_proto = htons(ETH_P_IP);
-
-		ip4h = vu_ip(flags_elem[0].in_sg[0].iov_base);
-		*ip4h = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_TCP);
-
-		th = vu_payloadv4(flags_elem[0].in_sg[0].iov_base);
-	} else {
-		eh->h_proto = htons(ETH_P_IPV6);
+	memcpy(eh.h_dest, c->guest_mac, sizeof(eh.h_dest));
-		ip6h = vu_ip(flags_elem[0].in_sg[0].iov_base);
-		*ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_TCP);
-		th = vu_payloadv6(flags_elem[0].in_sg[0].iov_base);
-	}
+	if (CONN_V4(conn))
+		ip4h = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_TCP);
+	else
+		ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_TCP);
-	memset(th, 0, sizeof(*th));
-	th->doff = sizeof(*th) / 4;
-	th->ack = 1;
Where are doff and ack now initialised?  Or were these already unneeded?
They are set in tcp_prepare_flags().

Thanks,
Laurent

    