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

[Jason Wang]

在 2021/5/11 下午4:33, Yuri Benditovich 写道:
> On Tue, May 11, 2021 at 9:50 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > 在 2021/5/11 下午12:42, Yuri Benditovich 写道:
> > > Signed-off-by: Yuri Benditovich <yuri.benditovich@xxxxxxxxxx>
> > > ---
> > >    drivers/net/tun.c | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > > index 84f832806313..a35054f9d941 100644
> > > --- a/drivers/net/tun.c
> > > +++ b/drivers/net/tun.c
> > > @@ -2812,7 +2812,7 @@ static int set_offload(struct tun_struct *tun, unsigned long arg)
> > >                        arg &= ~(TUN_F_TSO4|TUN_F_TSO6);
> > >                }
> > >
> > > -             arg &= ~TUN_F_UFO;
> > > +             arg &= ~(TUN_F_UFO|TUN_F_USO);
> >
> > It looks to me kernel doesn't use "USO", so TUN_F_UDP_GSO_L4 is a better
> > name for this
> No problem, I can change it in v2
>
>   and I guess we should toggle NETIF_F_UDP_GSO_l4 here?
>
> No, we do not, because this indicates only the fact that the guest can
> send large UDP packets and have them splitted to UDP segments.


Actually the reverse. The set_offload() controls the tuntap TX path 
(guest RX path).

When VIRTIO_NET_F_GUEST_XXX was not negotiated, the corresponding netdev 
features needs to be disabled. When host tries to send those packets to 
guest, it needs to do software segmentation.

See virtio_net_apply_guest_offloads().

There's currently no way (or not need) to prevent tuntap from receiving 
GSO packets.

Thanks


> > And how about macvtap?
> We will check how to do that for macvtap. We will send a separate
> patch for macvtap or ask for advice.
>
> > Thanks
> >
> >
> > >        }
> > >
> > >        /* This gives the user a way to test for new features in future by