Re: [PATCH v6 1/3] udp_vu: Allow virtqueue elements with multiple iovec entries

3 Apr 2026

On 4/3/26 13:53, Stefano Brivio wrote:
...
On Wed,  1 Apr 2026 21:23:24 +0200
Laurent Vivier  wrote:
...
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 
---
  udp_vu.c | 29 ++++++++++++++---------------
  1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/udp_vu.c b/udp_vu.c
index 30af64034516..5608a3a96ff5 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -64,30 +64,25 @@ 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[*cnt];
Variable-length Arrays (VLAs) are allowed starting from C99 but we
should really really avoid them. If 'cnt' is big enough, we risk
writing all over the place. That's the main reason why they were more
or less banned from the Linux kernel some years ago and eventually
eradicated:
I can use alloca() if you prefer ;)
...
https://lore.kernel.org/lkml/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1R...
https://lore.kernel.org/lkml/20181028172401.GA41102@beast/
Can we use VIRTQUEUE_MAX_SIZE as upper bound like udp_vu_sock_to_tap()
does?
Yes, but the idea here is we have always *cnt < VIRTQUEUE_MAX_SIZE
(because the value comes from vu_collect() and in vu_collect():
*in_sg (&iov_cnt or *cnt) < max_in_sg (ARRAY_SIZE(iov_vu) or VIRTQUEUE_MAX_SIZE or 1024)

And vu_collect(), in this case, sets generally *in_sg to a value lower than 44:
we want to create a frame of ETH_MAX_MTU by coalescing kernel buffers of size ETH_FRAME_LEN)

For me 16 kB on the stack is a lot of memory (but I started programming on a 48 kB RAM 
computer...).

Thanks,
Laurent

    