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

[Yuri Benditovich]

On Wed, May 12, 2021 at 4:33 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
>
> 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).

The set_offloads does 2 things:
1. At the initialization time qemu probes set_offload(something) to
check which features are supported by TAP/TUN.
2. Later it configures the guest RX path according to guest's needs/capabilities
Typical initialization sequence is (in case the QEMU supports USO feature):
TAP/TUN set offload 11 (probe for UFO support)
TAP/TUN set offload 21 (probe for USO support)
TAP/TUN set offload 0
...
TAP/TUN set offload 7 (configuration of offloads according to GUEST features)

This series of patches is for VIRTIO_NET_F_HOST_USO only, virtio-net
features like VIRTIO_NET_F_GUEST_USO_(4/6/whatever) are not defined in
the spec yet.

>
> 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
>