On 17-01-09 03:24 PM, Michael S. Tsirkin wrote: > 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? >
hmm in many of the drivers it is convention to use the mtu to set the rx buffer sizes and a receive side max length filter. For example in the Intel drivers if a packet with length greater than MTU + some headroom is received we drop it. I guess in the networking stack RX path though nothing forces this and virtio doesn't have any code to drop packets on rx size. In virtio I don't see any existing case currently. In the XDP case though we need to ensure packets fit in a page for the time being which is why I was looking at this code and generated this patch. >> 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. OK. By the way were do I get the latest source I see the published virtio1.0 at the oasis-open.org site but it doesn't mention the MTU logic. > >> 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. > Yep. I'm fine with calling it a misconfiguration if the guest reduces the MTU and the host continues to send packets @ advertised MTU.
