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

On Wed, May 12, 2021 at 4:33 AM Jason Wang [off-list ref] wrote:

在 2021/5/11 下午4:33, Yuri Benditovich 写道:

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On Tue, May 11, 2021 at 9:50 AM Jason Wang [off-list ref] wrote:

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在 2021/5/11 下午12:42, Yuri Benditovich 写道:

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Signed-off-by: Yuri Benditovich <redacted>
---
   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

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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.

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Thanks

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       }

       /* This gives the user a way to test for new features in future by

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