Re: [PATCH v2 1/2] tcp_vu: vu_pad() expects l2 length

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 23 Feb 2026 15:10:26 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> tcp_vu_hdrlen() returns a length that includes VNET_HLEN (the virtio
> net header),

No need for this patch/series but:

/**
 * tcp_vu_hdrlen() - return the size of the header in level 2 frame (TCP)
 * @v6:		Set for IPv6 packet
 *
 * Return: return the size of the header
 */

...can you fix that at some point? :)

It's really not clear what it returns. I assumed "level 2" meant
Layer-2 length (802.3-like), without virtio-net stuff, hence this issue.

> but vu_pad() expects the Layer-2 frame length, which
> should not include the virtio header. Passing the inflated length
> means short frames aren't padded to the minimum 60-byte Ethernet
> frame size (ETH_ZLEN).
> 
> Subtract VNET_HLEN from hdrlen when computing the l2 length passed
> to vu_pad() in both tcp_vu_send_flag() and tcp_vu_data_from_sock().
> 
> 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 | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/tcp_vu.c b/tcp_vu.c
> index 3847a4e92087..2c9ddba0c9a4 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -71,8 +71,8 @@ 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;
>  	struct vu_virtq_element flags_elem[2];
> +	size_t optlen, hdrlen, l2len;
>  	struct ipv6hdr *ip6h = NULL;
>  	struct iphdr *ip4h = NULL;
>  	struct iovec flags_iov[2];
> @@ -137,7 +137,8 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  	tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload,
>  			 NULL, seq, !*c->pcap);
>  
> -	vu_pad(&flags_elem[0].in_sg[0], hdrlen + optlen);
> +	l2len = optlen + hdrlen - VNET_HLEN;
> +	vu_pad(&flags_elem[0].in_sg[0], l2len);
>  
>  	if (*c->pcap)
>  		pcap_iov(&flags_elem[0].in_sg[0], 1, VNET_HLEN);
> @@ -446,6 +447,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  		int buf_cnt = head[i + 1] - head[i];
>  		ssize_t dlen = iov_size(iov, buf_cnt) - hdrlen;
>  		bool push = i == head_cnt - 1;
> +		size_t l2len;
>  
>  		vu_set_vnethdr(vdev, iov->iov_base, buf_cnt);
>  
> @@ -457,7 +459,8 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  		tcp_vu_prepare(c, conn, iov, buf_cnt, &check, !*c->pcap, push);
>  
>  		/* Pad first/single buffer only, it's at least ETH_ZLEN long */
> -		vu_pad(iov, dlen + hdrlen);
> +		l2len = dlen + hdrlen - VNET_HLEN;
> +		vu_pad(iov, l2len);
>  
>  		if (*c->pcap)
>  			pcap_iov(iov, buf_cnt, VNET_HLEN);

-- 
Stefano