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

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Message-ID: <74f6f0ea-6f2d-4520-b103-d4388a0916d6@redhat.com>
Date: Thu, 19 Jun 2025 19:27:34 +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 5:46 PM, Akihiko Odaki wrote:
> On 2025/06/19 23:52, Paolo Abeni wrote:
>> 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.
> 
> This explanation makes sense. In that case I suggest:
> - creating a new function named tun_vnet_hdr_tnl_get() and
> - passing vnet_hdr_sz to tun_vnet_hdr_tnl_to_skb()
> 
> tun_vnet.h contains the virtio-related logic for better code 
> organization and reuse with tap.c. tap.c can reuse the conditionals on 
> vnet_hdr_sz when tap.c gains the UDP tunneling support.

Instead of repeating the test twice (in both tun_vnet_hdr_tnl_to_skb()
and tun_vnet_hdr_tnl_to_skb(), what about creating a new helper:

tun_vnet_hdr_guest_features(unsigned int vnet_hdr_len)

encapsulating the above logic? That will make also easier to move to the
'correct' solution of having the tun/tap devices aware of the features
negotiated in the guest -> host direction.

/P