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

From: Maxime Coquelin
Subject: Re: [Qemu-devel] [RFC v2 0/3] virtio-net: Add support to MTU feature
Date: Fri, 18 Nov 2016 19:52:30 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

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
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.


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