Re: [Qemu-devel] [PATCH v1 0/1] Allow TAP to unnegotiate virtio-net header

[Jason Wang <jasowang@redhat.com>]

On 2019/2/16 上午1:46, Vincenzo Maffione wrote:
> Hi,
>    I was doing experiments with a custom paravirtualized net device,
> and I ran into a limitation of the TAP backend. I see from the kernel
> code that it is not possible to set the TAP virtio-net header
> length to something different from 10 or 12, which means that it
> is not possible to set it to 0. That's fine.
> The QEMU implementation of the TAP backend already supports
> the case where virtio-net header is set to 10 or 12 in kernel,
> but the emulated/paravirtualized net device is not using that:
> the TAP backend will just strip/prepend a zeroed header in this case
> (e.g., see the if statement in tap_receive_iov()).
> However, the tap_using_vnet_hdr() has an assert() that prevents
> this situation to happen, and I don't understand why. Maybe it
> is a leftover? I tried to remove the assert and by doing that
> my paravirtualized device was able to stop using the virtio-net
> header.


Hi:

If  I understand this correctly, your PV device is API compatible with 
TAP? Then you may just adding code to call TUNSETIFF without IFF_VNET_HDR?

Thanks


>
> Vincenzo Maffione (1):
>    net: tap: allow net frontends to un-negotiate virtio-net header
>
>   net/tap.c | 1 -
>   1 file changed, 1 deletion(-)
>