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

[Paolo Abeni <pabeni@redhat.com>]

On 6/19/25 7:41 PM, Paolo Abeni wrote:
> On 6/19/25 6:02 PM, Akihiko Odaki wrote:
>> On 2025/06/18 1:12, Paolo Abeni wrote:
>>> @@ -2426,7 +2453,17 @@ static int tun_xdp_one(struct tun_struct *tun,
>>>   	if (metasize > 0)
>>>   		skb_metadata_set(skb, metasize);
>>>   
>>> -	if (tun_vnet_hdr_to_skb(tun->flags, skb, gso)) {
>>> +	/*
>>> +	 * Assume tunnel offloads are enabled if the received hdr is large
>>> +	 * enough.
>>> +	 */
>>> +	if (READ_ONCE(tun->vnet_hdr_sz) >= TUN_VNET_TNL_SIZE &&
>>> +	    xdp->data - xdp->data_hard_start >= TUN_VNET_TNL_SIZE)
>>> +		features = NETIF_F_GSO_UDP_TUNNEL |
>>> +			   NETIF_F_GSO_UDP_TUNNEL_CSUM;
>>
>> xdp->data - xdp->data_hard_start may not represent the header size.
>>
>> struct tun_xdp_hdr is filled in vhost_net_build_xdp() in 
>> drivers/vhost/net.c. This function sets the two fields with 
>> xdp_prepare_buff(), but the arguments passed to xdp_prepare_buff() does 
>> not seem to represent the exact size of the header.
> 
> Indeed the xdp->data - xdp->data_hard_start range additionally contains
> some padding and eventually the xdp specific headroom. The problem is
> that both info are vhost_net specific and tun can't (or at least shoul)
> not be aware of them.
> 
> The only IMHO feasible refinement could be using xdp->data_meta instead
> of xdp->data when available. Alternatively I could drop entirely the test.

As the vhost_net padding is considerably larger than the largest
possible vnet_hdr_sz, I'll drop entirely the xdp check - so that I can
use the newly introduced tun_vnet_hdr_guest_features() helper here, too.

/P