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

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Message-ID: <CACGkMEtA6nHo2abbBjvfMrH-yfDhi1DJ_MXKtvcWumq-XNuu7A@mail.gmail.com>
Date: Tue, 17 Jun 2025 10:56:16 +0800
From: Jason Wang <jasowang@...hat.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, 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 Mon, Jun 16, 2025 at 6:43 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> 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).

This is why I feel strange when reading the patches. Everything would
be simplified if the offset is fixed.

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

Thanks