netdev - Re: [PATCH v4 net-next 7/8] tun: enable gso over UDP tunnel support.

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Message-ID: <4e0a0a37-9164-465a-b18b-7d97c88d444e@redhat.com>
Date: Thu, 19 Jun 2025 16:52:58 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>, netdev@...r.kernel.org
Cc: 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>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Yuri Benditovich <yuri.benditovich@...nix.com>
Subject: Re: [PATCH v4 net-next 7/8] tun: enable gso over UDP tunnel support.

On 6/19/25 4:42 PM, Akihiko Odaki wrote:
> On 2025/06/18 1:12, Paolo Abeni wrote:
>> @@ -1721,7 +1733,12 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>>   	if (tun->flags & IFF_VNET_HDR) {
>>   		int vnet_hdr_sz = READ_ONCE(tun->vnet_hdr_sz);
>>   
>> -		hdr_len = tun_vnet_hdr_get(vnet_hdr_sz, tun->flags, from, &gso);
>> +		if (vnet_hdr_sz >= TUN_VNET_TNL_SIZE)
>> +			features = NETIF_F_GSO_UDP_TUNNEL |
>> +				   NETIF_F_GSO_UDP_TUNNEL_CSUM;
> 
> I think you should use tun->set_features instead of tun->vnet_hdr_sz to 
> tell if these features are enabled.

This is the guest -> host direction. tun->set_features refers to the
opposite one. The problem is that tun is not aware of the features
negotiated in the guest -> host direction.

The current status (for baremetal/plain offload) is allowing any known
feature the other side send - if the virtio header is consistent.
This code follows a similar schema.

Note that using 'tun->set_features' instead of 'vnet_hdr_sz' the tun
driver will drop all the (legit) GSO over UDP packet sent by the guest
when the VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO has been negotiated and
VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO has not.

/P