Re: [Qemu-devel] [RFC v2 0/3] virtio-net: Add support to MTU feature

[Maxime Coquelin]

On 11/18/2016 07:15 PM, Aaron Conole wrote:
> Maxime Coquelin <address@hidden> writes:
>
> > This series implements Virtio spec update from Aaron Conole which
> > defines a way for the host to expose its max MTU to the guest.
> >
> > Changes since RFC v1:
> > ---------------------
> >  - Rebased on top of v2.8.0-rc0 (2.7.90)
> >  - Write MTU unconditionnaly in netcfg to avoid memory leak (Paolo)
> >  - Add host_mtu property to be able to disable the feature from QEMU
> >
> > Maxime Coquelin (3):
> >   vhost-user: Add new protocol feature MTU
> >   vhost-net: Add new MTU feature support
> >   virtio-net: Add MTU feature support
> >
> >  hw/net/vhost_net.c             | 11 +++++++++++
> >  hw/net/virtio-net.c            | 14 ++++++++++++++
> >  hw/virtio/vhost-user.c         | 11 +++++++++++
> >  include/hw/virtio/vhost.h      |  1 +
> >  include/hw/virtio/virtio-net.h |  1 +
> >  include/net/vhost_net.h        |  2 ++
> >  6 files changed, 40 insertions(+)
>
> I ran this with a VM, but it seems the offered maximum MTU was of value
> 0 - is this expected with this version?  How can I change the offered
> value?  Sorry, I'm not as familiar with QEMU/libvirt side of the world.

They way I implemented it, the MTU value is to be provided by
vhost-user process (e.g. OVS/DPDK). I added a Vhost protocol
feature for this. The sequence is:
1. Qemu send VHOST_USER_GET_PROTOCOL_FEATURES request
2. DPDK replies with providing supported features
3. If DPDK supports VHOST_USER_PROTOCOL_F_MTU, Qemu send
   VHOST_USER_GET_MTU resuest
4. DPDK replies with MTU value

Does that make sense?

Another possibility would be that we could directly pass the MTU value
to Qemu. It may be easier to implement, and to handle migration.
Problem is that if we do this, this is not the vSwitch that decides the
MTU to set.

Regards,
Maxime