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

[Michael S. Tsirkin]

On Tue, Nov 22, 2016 at 09:32:01AM -0500, Aaron Conole wrote:
> Maxime Coquelin <address@hidden> writes:
> 
> > On 11/22/2016 05:07 AM, Jason Wang wrote:
> >>
> >>
> >> 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:
> >>>>> 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?
> 
> I think qemu should just query the MTU at start time and then
> provide that as the value to the VM.  Why specify with command line
> option?

Passing it on command line is an easy way to make sure we can migrate
the VM correctly.  Also, if we get it from the backend, this requires
backend changes.  In particular, tun/macvtap will need new ioctls.  So
generally that's more work.

>  Seems to me like an easy way to get out of sync.

If we send it to the backend, that has a chance to check
mtu and disconnect on error.

> > The spec says the MTU must not be modified by the device once it has
> > been set.
> 
> +1 - we don't have an exchange or negotiation mechanism for this.  That
> would require additional bits and communication, and it seems like it
> isn't worth it.
> 
> > I think it would require a device reset if MTU came to change.
> 
> It's just too much work for not enough gain.  Guest can change MTU all
> it wants.  Host should just know what it will limit to the guest from
> the beginning.  Maybe I'm too simple, though.
> 
> -Aaron

I agree it's a good start.

-- 
MST