On 17-01-09 07:55 PM, Michael S. Tsirkin wrote: > On Mon, Jan 09, 2017 at 07:30:34PM -0800, John Fastabend wrote: >> 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 don't understand the question. Look at add_recvbuf_small, > it adds a tiny buffer for head and then the skb. >
Specifically this seems to fail with mergeable buffers disabled On the host: # ip link set dev tap0 mtu 9000 # ping 22.2 -s 2048 On the guest: # insmod ./drivers/net/virtio_net.ko # ip link set dev eth0 mtu 9000 With mergeable buffers enabled no problems it works as I expect at least. > >> 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 > > There's always a single buffer there. > BTW it was always a legacy path but if it's now important for people we > should probably check ANY_LAYOUT and put header linearly with the packet > if there. >
