[PATCH v8 1/3] udp_vu: Allow virtqueue elements with multiple iovec entries

Laurent Vivier
2026-04-16T16:09:32Z

The previous code assumed a 1:1 mapping between virtqueue elements and
iovec entries (enforced by an assert). Drop that assumption to allow
elements that span multiple iovecs: track elem_used separately by
walking the element list against the iov count returned after padding.
This also fixes vu_queue_rewind() and vu_flush() to use the element
count rather than the iov count.

Use iov_tail_clone() in udp_vu_sock_recv() to handle header offset,
replacing the manual base/len adjustment and restore pattern.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
---
udp_vu.c | 31 ++++++++++++++++---------------
1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/udp_vu.c b/udp_vu.c
index 5464917a0153..ef8e60cc390a 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -66,30 +66,25 @@ static size_t udp_vu_hdrlen(bool v6)
*/
static ssize_t udp_vu_sock_recv(struct iovec *iov, size_t *cnt, int s, bool v6)
{
+ struct iovec msg_iov[VIRTQUEUE_MAX_SIZE];
struct msghdr msg = { 0 };
+ struct iov_tail payload;
size_t hdrlen, iov_used;
ssize_t dlen;

/* compute L2 header length */
hdrlen = udp_vu_hdrlen(v6);

- /* reserve space for the headers */
- assert(iov[0].iov_len >= MAX(hdrlen, ETH_ZLEN + VNET_HLEN));
- iov[0].iov_base = (char *)iov[0].iov_base + hdrlen;
- iov[0].iov_len -= hdrlen;
+ payload = IOV_TAIL(iov, *cnt, hdrlen);

- /* read data from the socket */
- msg.msg_iov = iov;
- msg.msg_iovlen = *cnt;
+ msg.msg_iov = msg_iov;
+ msg.msg_iovlen = iov_tail_clone(msg.msg_iov, payload.cnt, &payload);

+ /* read data from the socket */
dlen = recvmsg(s, &msg, 0);
if (dlen < 0)
return -1;

- /* restore the pointer to the headers address */
- iov[0].iov_base = (char *)iov[0].iov_base - hdrlen;
- iov[0].iov_len += hdrlen;
-
iov_used = iov_skip_bytes(iov, *cnt,
MAX(dlen + hdrlen, VNET_HLEN + ETH_ZLEN),
NULL);
@@ -202,7 +197,7 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
}

for (i = 0; i < n; i++) {
- unsigned elem_cnt, elem_used;
+ unsigned elem_cnt, elem_used, j, k;
size_t iov_cnt;
ssize_t dlen;

@@ -212,15 +207,21 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
if (elem_cnt == 0)
break;

- assert((size_t)elem_cnt == iov_cnt); /* one iovec per element */
-
dlen = udp_vu_sock_recv(iov_vu, &iov_cnt, s, v6);
if (dlen < 0) {
vu_queue_rewind(vq, elem_cnt);
break;
}

- elem_used = iov_cnt; /* one iovec per element */
+ elem_used = 0;
+ for (j = 0, k = 0; k < iov_cnt && j < elem_cnt; j++) {
+ size_t iov_still_needed = iov_cnt - k;
+
+ if (elem[j].in_num > iov_still_needed)
+ elem[j].in_num = iov_still_needed;
+ k += elem[j].in_num;
+ elem_used++;
+ }

/* release unused buffers */
vu_queue_rewind(vq, elem_cnt - elem_used);
--
2.53.0