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

From: Jason Wang
Subject: Re: [Qemu-devel] [RFC v2 0/3] virtio-net: Add support to MTU feature
Date: Tue, 22 Nov 2016 12:07:38 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 2016年11月22日 00:23, Michael S. Tsirkin wrote:
On Fri, Nov 18, 2016 at 02:21:54PM -0500, Aaron Conole wrote:
Maxime Coquelin <address@hidden> writes:

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:
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.

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.

Or vice-versa? E.g qemu need to be notified if the MTU of tap or macvtap were changed?

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