netdev - Re: [Q] packet truncated after enabling ip

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Date:   Wed, 14 Sep 2022 11:46:56 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [Q] packet truncated after enabling ip_forward for virtio-net in guest

On Tue, Sep 13, 2022 at 11:43 PM Wei Yang <richard.weiyang@...il.com> wrote:
>
> 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,

Note that form device POV, it should be VIRTIO_NET_F_GUEST_TSOX.

> 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?

Note that if you are using tuntap, you should refert driver/net/tun.c
instead of tap.c. Where it calls netdev_update_features() that will
change the TX offloading.

Thanks

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