Re: [PATCH 2/2] tcp_vu, udp_vu: Account for virtio net header in minimum frame size

On Thu, 12 Feb 2026 12:39:32 +0100 Laurent Vivier wrote:

In the vhost-user paths, the buffers provided by the virtio queue include the virtio net header (VNET_HLEN) prepended to the Ethernet frame. The minimum size checks using ETH_ZLEN must therefore account for this additional header length, otherwise we underestimate the minimum buffer size needed.

Oops, thanks for fixing this.

Use ETH_ZLEN + VNET_HLEN instead of bare ETH_ZLEN in vu_collect() calls and the corresponding ASSERT() checks.

Fixes: 0cb8f9003654 ("tcp, udp: Pad batched frames for vhost-user modes to 60 bytes (802.3 minimum)") Signed-off-by: Laurent Vivier --- tcp_vu.c | 8 ++++---- udp_vu.c | 2 +- 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/tcp_vu.c b/tcp_vu.c index f7bda4943e43..2d593d534d68 100644 --- a/tcp_vu.c +++ b/tcp_vu.c @@ -90,12 +90,12 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) vu_set_element(&flags_elem[0], NULL, &flags_iov[0]);

elem_cnt = vu_collect(vdev, vq, &flags_elem[0], 1, - MAX(hdrlen + sizeof(*opts), ETH_ZLEN), NULL); + MAX(hdrlen + sizeof(*opts), ETH_ZLEN + VNET_HLEN), NULL);

I'm applying this now but, perhaps as a follow-up, should we consider to clarify further the arguments to vu_collect(), and perhaps related sizes? I'm not the most indicated person to do this I guess, as I keep getting them wrong. :( In this case, I really thought VNET_HLEN would be added internally, because of: * @size: Maximum size of the data in the frame -- Stefano