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

[Jason Wang <jasowang@redhat.com>]

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 <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.

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