Linux Virtualization - May 2021 - [PATCH 4/4] tun: indicate support for USO feature

On Fri, May 14, 2021 at 4:35 AM Willem de Bruijn
<willemdebruijn.kernel at gmail.com> 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

On Fri, May 14, 2021 at 10:16 AM Jason Wang <jasowang at redhat.com>
wrote:
>
> On Fri, May 14, 2021 at 4:35 AM Willem de Bruijn
> <willemdebruijn.kernel at gmail.com> 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
Is it possible that the virtio-net does not disable UFO offload?
IMO it sets NETIF_F_LRO too bravely.
>
> >
> > 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
>