On Mon, Jan 09, 2017 at 03:13:15PM -0800, John Fastabend wrote: > On 17-01-09 03:05 PM, Michael S. Tsirkin wrote: > > On Thu, Jan 05, 2017 at 11:09:14AM +0800, Jason Wang wrote: > >> > >> > >> On 2017年01月05日 02:57, John Fastabend wrote: > >>> [...] > >>> > >>>> On 2017年01月04日 00:48, John Fastabend wrote: > >>>>> On 17-01-02 10:14 PM, Jason Wang wrote: > >>>>>> On 2017年01月03日 06:30, John Fastabend wrote: > >>>>>>> XDP programs can not consume multiple pages so we cap the MTU to > >>>>>>> avoid this case. Virtio-net however only checks the MTU at XDP > >>>>>>> program load and does not block MTU changes after the program > >>>>>>> has loaded. > >>>>>>> > >>>>>>> This patch sets/clears the max_mtu value at XDP load/unload time. > >>>>>>> > >>>>>>> Signed-off-by: John Fastabend <john.r.fastab...@intel.com> > >>>>>>> --- > >>> [...] > >>> > >>>>> OK so this logic is a bit too simply. When it resets the max_mtu I > >>>>> guess it > >>>>> needs to read the mtu via > >>>>> > >>>>> virtio_cread16(vdev, ...) > >>>>> > >>>>> or we may break the negotiated mtu. > >>>> Yes, this is a problem (even use ETH_MAX_MTU). We may need a method to > >>>> notify > >>>> the device about the mtu in this case which is not supported by virtio > >>>> now. > >>> Note this is not really a XDP specific problem. The guest can change the > >>> MTU > >>> after init time even without XDP which I assume should ideally result in a > >>> notification if the MTU is negotiated. > >> > >> Yes, Michael, do you think we need add some mechanism to notify host about > >> MTU change in this case? > >> > >> Thanks > > > > Why does host care? > > > > Well the guest will drop packets after mtu has been reduced.
I didn't know. What place in code does this? > Although the guest > by reducing its MTU in some sense must expect this. Likewise if the host were > to change MTU after virtio_net probe time the guest would not learn about it. The spec explicitly disallows this last one. > I think at best negotiating the mtu is just a hint? If system _really_ cares > we could use lldp or some other out of band mechanism to learn/set/adjust MTU > on both systems and it would be more robust. I'm not actually convinced this > is a problem in bare metal systems we have the same issue with physical > switches and solve it out of band via configuration, protocols, etc. > > .John ATM we don't have negotiation in virtio, just a max mtu limit. This doesn't free guest from configuring mtu correctly, just helps it avoid doing something clearly bogus. -- MST
