netdev - [Q] packet truncated after enabling ip_forward for virtio-net in guest

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Message-ID: <CADZGycYUH=j80zmJVr7dfVtoJ+BrbAEPJE8Nvf3HR5oimJR+UQ@mail.gmail.com>
Date:   Tue, 13 Sep 2022 23:42:51 +0800
From:   Wei Yang <richard.weiyang@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, jasowang@...hat.com
Cc:     netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: [Q] packet truncated after enabling ip_forward for virtio-net in guest

Hi, I am running a guest with vhost-net as backend. After I enable
ip_forward, the packet received is truncated.

Host runs a 5.10 kernel, while guest kernel is v5.11 which doesn't
include this commit:

  virtio-net: use NETIF_F_GRO_HW instead of NETIF_F_LRO

After applying this commit, the issue is gone. I guess the reason is
this device doesn't have NETIF_F_GRO_HW set, so
virtnet_set_guest_offloads is not called.

I am wondering why packet is truncated without this fix. I follow
virtnet_set_guest_offloads and just see virtio_net_handle_ctrl in qemu
handles VIRTIO_NET_CTRL_GUEST_OFFLOADS. Since we use a tap dev, then I
follow tap_fd_set_offload to ioctl(fd, TUNSETOFFLOAD, offload).

But I am lost here. tap_ioctl -> set_offload(). Since we use a normal
tap device instead of ipvtap/macvtap, update_features is empty. So I
don't get how the device's behavior is changed after set LRO.

Do I follow the wrong path? Any suggestions on investigation?

I'd appreciate it if someone could give a hint :-)