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

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 12 Feb 2026 12:39:32 +0100
Laurent Vivier <lvivier@redhat.com> 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.
> 
> 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 <lvivier@redhat.com>
> ---
>  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);
>  	if (elem_cnt != 1)
>  		return -1;
>  
>  	ASSERT(flags_elem[0].in_sg[0].iov_len >=
> -	       MAX(hdrlen + sizeof(*opts), ETH_ZLEN));
> +	       MAX(hdrlen + sizeof(*opts), ETH_ZLEN + VNET_HLEN));
>  
>  	vu_set_vnethdr(vdev, flags_elem[0].in_sg[0].iov_base, 1);
>  
> @@ -208,7 +208,7 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>  
>  		cnt = vu_collect(vdev, vq, &elem[elem_cnt],
>  				 VIRTQUEUE_MAX_SIZE - elem_cnt,
> -				 MAX(MIN(mss, fillsize) + hdrlen, ETH_ZLEN),
> +				 MAX(MIN(mss, fillsize) + hdrlen, ETH_ZLEN + VNET_HLEN),
>  				 &frame_size);
>  		if (cnt == 0)
>  			break;
> @@ -302,7 +302,7 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn,
>  	/* we guess the first iovec provided by the guest can embed
>  	 * all the headers needed by L2 frame, including any padding
>  	 */
> -	ASSERT(iov[0].iov_len >= MAX(hdrlen, ETH_ZLEN));
> +	ASSERT(iov[0].iov_len >= MAX(hdrlen, ETH_ZLEN + VNET_HLEN));

This triggers in the passt_vu_in_ns/tcp, "TCP/IPv4: host to guest: big
transfer" test case, that is, the first time we connect to the guest
(probably on the initial SYN segment).

I didn't check why.

I applied 1/2, but not this one.

-- 
Stefano