On 17-01-09 06:51 PM, Michael S. Tsirkin wrote: > On Tue, Jan 10, 2017 at 10:29:39AM +0800, Jason Wang wrote: >> >> >> On 2017年01月10日 07:58, Michael S. Tsirkin wrote: >>> On Mon, Jan 09, 2017 at 03:49:27PM -0800, John Fastabend wrote: >>>> 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. >>> I'd say just look at the hardware max mtu. Ignore the configured mtu. >>> >>> >> >> Does this work for small buffers consider it always allocate skb with size >> of GOOD_PACKET_LEN? > > Spec says hardware won't send in packets > max mtu in config space. > >> I think in any case, we should limit max_mtu to >> GOOD_PACKET_LEN for small buffers. >> >> Thanks > > XDP seems to have a bunch of weird restrictions, I just > do not like it that the logic spills out to all drivers. > What if someone decides to extend it to two pages in the future? > Recode it all in all drivers ... > > Why can't net core enforce mtu? >
OK I agree I'll put most the logic in rtnetlink.c when the program is added or removed. But, I'm looking at the non-XDP receive_small path now and wondering how does multiple buffer receives work (e.g. packet larger than GOOD_PACKET_LEN?) I think this is what Jason is looking at as well? The mergeable case clearly looks at num_bufs in the descriptor to construct multi-buffer packets but nothing like that exists in the small_receive path as best I can tell. .John
