netdev - Re: [PATCH net v5 1/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST

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Message-ID: <25b05194-63cd-4265-8d2c-e174d801fc3a@redhat.com>
Date: Thu, 13 Nov 2025 15:39:35 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, netdev@...r.kernel.org
Cc: Richard Weinberger <richard@....at>,
 Anton Ivanov <anton.ivanov@...bridgegreys.com>,
 Johannes Berg <johannes@...solutions.net>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Jason Wang <jasowang@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, "Michael S. Tsirkin" <mst@...hat.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 Simon Horman <horms@...nel.org>, Jiri Pirko <jiri@...nulli.us>,
 Alvaro Karsz <alvaro.karsz@...id-run.com>, linux-um@...ts.infradead.org,
 virtualization@...ts.linux.dev
Subject: Re: [PATCH net v5 1/2] virtio-net: correct hdr_len handling for
 VIRTIO_NET_F_GUEST_HDRLEN

On 11/11/25 12:12 PM, Xuan Zhuo wrote:
> The commit be50da3e9d4a ("net: virtio_net: implement exact header length
> guest feature") introduces support for the VIRTIO_NET_F_GUEST_HDRLEN
> feature in virtio-net.
> 
> This feature requires virtio-net to set hdr_len to the actual header
> length of the packet when transmitting, the number of
> bytes from the start of the packet to the beginning of the
> transport-layer payload.
> 
> However, in practice, hdr_len was being set using skb_headlen(skb),
> which is clearly incorrect. This commit fixes that issue.
> 
> Fixes: be50da3e9d4a ("net: virtio_net: implement exact header length guest feature")
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>

IMHO this looks like more a new feature - namely,
VIRTIO_NET_F_GUEST_HDRLEN support - than a fix.

[...]
> @@ -2361,7 +2362,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
>  	if (vnet_hdr_sz &&
>  	    virtio_net_hdr_from_skb(skb, h.raw + macoff -
>  				    sizeof(struct virtio_net_hdr),
> -				    vio_le(), true, 0)) {
> +				    vio_le(), true, false, 0)) {
>  		if (po->tp_version == TPACKET_V3)
>  			prb_clear_blk_fill_status(&po->rx_ring);
>  		goto drop_n_account;
To reduce the diffstat, what about creating a __virtio_net_hdr_from_skb()
variant (please find a better name) allowing the extra `hdrlen_negotiated`
argument, define virtio_net_hdr_from_skb() as a wrapper of such helper
withthe extra arg == false, and use the helper in the few places that
really could use hdrlen?