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

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Message-ID: <81b7e1a5-3c7c-484d-a588-de67eb907bbf@redhat.com>
Date: Fri, 20 Jun 2025 17:51:59 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, netdev@...r.kernel.org
Cc: 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>,
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: Re: [PATCH v4 net-next 7/8] tun: enable gso over UDP tunnel support.

On 6/20/25 4:40 PM, Willem de Bruijn wrote:
> Paolo Abeni wrote:
>> @@ -1698,7 +1700,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>>  	struct sk_buff *skb;
>>  	size_t total_len = iov_iter_count(from);
>>  	size_t len = total_len, align = tun->align, linear;
>> -	struct virtio_net_hdr gso = { 0 };
>> +	struct virtio_net_hdr_v1_hash_tunnel hdr;
> 
> Not for this series.
> 
> But one day virtio will need a policy on how multiple optional
> features can be composed, and simple APIs to get to those optional
> headers. Perhaps something like skb extensions.
> 
> Now, each new extention means adding yet another struct and updating
> all sites that access it.
> 
> A minimal rule may be that options can be entirely independent, but
> if they exist at least their headers are always in a fixed order.
> Which is already implied by the current extensions, i.e., hash comes
> before tunnel if present.

If I read correctly, you are suggesting that negotiating tunnel and not
hash would yield this layout:

< basic vnet hdr> <tnl fields>

with no gaps/data between the basic header fields and the tunnel-related
one. Am I correct?

This has been discussed in the previous revisions, and a recent
specification update explicitly states differently: with tunnel support
and without hash report the only possible layout is:

< basic vnet hdr> <hash report field (unused)> <tnl fields>

Since it's in the spec it's too late to change it, unless we add yet
another feature for that. I'm gladly leaving that joy and fun to someone
else:)

FTR the initial revisions of this series, before I stumbled upon the
mentioned spec change, followed the schema you mentioned.

>> @@ -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;
> 
> Maybe a helper virtio_net_has_opt_tunnel(), to encapsulate whatever
> conditions have to be met. As those conditions are not obvious.
> 
> Especially if needed in multiple locations. Not sure if that is the
> case here, I have not checked that.

Yep, as an outcome of Ajihiko's review I'm encapsulation the above in
a new helper - tun_vnet_hdr_guest_features() to be more generic.

>> @@ -2812,6 +2849,8 @@ static void tun_get_iff(struct tun_struct *tun, struct ifreq *ifr)
>>  
>>  }
>>  
>> +#define PLAIN_GSO (NETIF_F_GSO_UDP_L4 | NETIF_F_TSO | NETIF_F_TSO6)
>> +
> 
> Minor/subjective: prefer const unsigned int at function scope over untyped
> file scope macros.

Unless it's blocking I would keep the current code here.

>> +static inline int
>> +tun_vnet_hdr_tnl_to_skb(unsigned int flags, netdev_features_t features,
>> +			struct sk_buff *skb,
>> +			const struct virtio_net_hdr_v1_hash_tunnel *hdr)
>> +{
>> +	return virtio_net_hdr_tnl_to_skb(skb, hdr,
>> +					 !!(features & NETIF_F_GSO_UDP_TUNNEL),
>> +					 !!(features & NETIF_F_GSO_UDP_TUNNEL_CSUM),
> 
> Double exclamation points not needed. Compiler does the right thing
> when arguments are of type bool.

Will drop in the next revision.

Thanks!

Paolo