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

[Vincenzo Maffione]

Hi Jason,
  Thanks for the quick reply.

My PV device (to be open sourced soon) uses the QEMU net backend interface,
in a way similar to virtio-net.
For example it uses qemu_set_offload(), qemu_has_vnet_hdr_len(),
qemu_using_vnet_hdr(), qemu_send_packet(), etc.
This means that the device itself does not know which net backend is using,
of course.
In addition to TAP, also the netmap backend supports the virtio-net header,
and so the PV device will work with both.
Regarding the virtio-net header, the netmap backend is more flexible
because you can change the virtio-net header length as you wish (choosing
between 0, 10 and 12 bytes).

My problem cannot be solved by making sure that TAPs are opened without
IFF_VNET_HDR. What I want instead
is to dynamically change the length of the virtio-net header that the
backend accepts, switching between 12 bytes (virtio 1.0 header) and 0 (no
header). By "dynamically" I mean while the guest (and the device) is
running. This is possible and safe if we get rid of that assert().
The virtio-net device does not need that flexibility (once you enable the
vtnet header, you never disable that again), so I guess that's why you have
that assert().
My PV device can be reconfigured on the fly in such a way it may or not may
be aware of the virtio-net header, and as a consequence it may need to call
qemu_using_vnet_hdr() with true or false (multiple times).

Does it make sense?

Cheers,
  Vincenzo


Il giorno lun 18 feb 2019 alle ore 05:19 Jason Wang <address@hidden>
ha scritto:

>
> 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(-)
> >
>


-- 
Vincenzo