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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 18, 2016 at 02:21:54PM -0500, Aaron Conole wrote:
> Maxime Coquelin <maxime.coquelin@redhat.com> writes:
> 
> > On 11/18/2016 07:15 PM, Aaron Conole wrote:
> >> Maxime Coquelin <maxime.coquelin@redhat.com> 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?
> 
> In the case of a vhost-user backed port, yes (so for instance, if I use
> ovs+dpdk vhost-user in client or server mode).  However, what about the
> non-dpdk case, where I still use a virtio-net driver in kernel and want
> to have it backed with, say, a tap device in the host attached to
> virbr0 (or some other bridge).  It should still pull the mtu from that
> device and offer it, I think.
> 
> > 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.
> 
> Might be better to determined the mtu by looking at what actually
> provides the back-end for the networking.
> 
> > Regards,
> > Maxime

Right. So in case it's not vhost-user, I would say it has to
be specified from QEMU command line.
It's probably easier to do the same everywhere, and just send
the MTU from qemu to backend.


-- 
MST