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

[Akihiko Odaki <akihiko.odaki@daynix.com>]

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.