netdev - Re: [PATCH RFC v3 7/8] tun: enable gso over UDP tunnel support.

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Message-ID: <a1a4b28d-2bc2-4968-9e79-18ced60f8cb0@redhat.com>
Date: Mon, 16 Jun 2025 12:43:28 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, 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 RFC v3 7/8] tun: enable gso over UDP tunnel support.

On 6/16/25 12:17 PM, Paolo Abeni wrote:
> Anyhow I now see that keeping the UDP GSO related fields offset constant
> regardless of VIRTIO_NET_F_HASH_REPORT would remove some ambiguity from
> the relevant control path.
> 
> I think/hope we are still on time to update the specification clarifying
> that, but I'm hesitant to take that path due to the additional
> (hopefully small) overhead for the data path - and process overhead TBH.
> 
> On the flip (positive) side such action will decouple more this series
> from the HASH_REPORT support.

Whoops, I did not noticed:

https://lore.kernel.org/virtio-comment/20250401195655.486230-1-kshankar@marvell.com/
(virtio-spec commit 8d76f64d2198b34046fbedc3c835a6f75336a440 and
specifically:

"""
When calculating the size of \field{struct virtio_net_hdr}, the driver
MUST consider all the fields inclusive up to \field{padding_reserved_2}
// ...
i.e. 24 bytes if VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO is negotiated or up to
\field{padding_reserved}
"""

that is, UDP related fields are always in a fixed position, regardless
of VIRTIO_NET_F_HASH_REPORT (and other previous discussion not captured
in the spec clearly enough).

TL;DR: please scratch my previous comment above, I'll update the patches
accordingly (fixed UDP GSO field offset).

/P