netdev - Re: [PATCH 4/4] tun: indicate support for USO feature

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Date:   Thu, 13 May 2021 16:34:57 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Yuri Benditovich <yuri.benditovich@...nix.com>,
        Yan Vugenfirer <yan@...nix.com>, davem <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, mst <mst@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH 4/4] tun: indicate support for USO feature

> > But surprisingly when TUN receives TUN_F_UFO it does not propagate it
> > anywhere, there is no corresponding NETIF flag.
>
> (It looks like I drop the community and other ccs accidentally, adding
> them back and sorry)
>
> Actually, there is one, NETIF_F_GSO_UDP.
>
> Kernel used to have NETIF_F_UFO, but it was removed due to bugs and
> the lack of real hardware support. Then we found it breaks uABI, so
> Willem tries to make it appear for userspace again, and then it was
> renamed to NETIF_F_GSO_UDP.
>
> But I think it's a bug that we don't proporate TUN_F_UFO to NETIF
> flag, this is a must for the driver that doesn't support
> VIRTIO_NET_F_GUEST_UFO. I just try to disable all offloads and
> mrg_rxbuf, then netperf UDP_STREAM from host to guest gives me bad
> length packet in the guest.
>
> Willem, I think we probably need to fix this.

We had to add back support for the kernel to accept UFO packets from
userspace over tuntap.

The kernel does not generate such packets, so a guest should never be
concerned of receiving UFO packets.

Perhaps i'm misunderstanding the problem here.