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

[Jason Wang]

On Fri, May 14, 2021 at 4:35 AM Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
>
> > > 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.

That's my feeling as well.

But when I:

1) turn off all guest gso feature and mrg rx buffers, in this case
virtio-net will only allocate 1500 bytes for each packet
2) doing netperf (UDP_STREAM) from local host to guest, I see packet
were truncated in the guest

>
> Perhaps i'm misunderstanding the problem here.
>

I will re-check and get back to you.
(probably need a while since I will not be online for the next week).

Thanks