On 5/16/18 1:24 AM, Jason Wang wrote: > > > On 2018年05月16日 11:51, David Ahern wrote: >> Hi Jason: >> >> I am trying to test MTU changes to the BPF fib_lookup helper and seeing >> something odd. Hoping you can help. >> >> I have a VM with multiple virtio based NICs and tap backends. I install >> the xdp program on eth1 and eth2 to do forwarding. In the host I send a >> large packet to eth1: >> >> $ ping -s 1500 9.9.9.9 >> >> >> The tap device in the host sees 2 packets: >> >> $ sudo tcpdump -nv -i vm02-eth1 >> 20:44:33.943160 IP (tos 0x0, ttl 64, id 58746, offset 0, flags [+], >> proto ICMP (1), length 1500) >> 10.100.1.254 > 9.9.9.9: ICMP echo request, id 17917, seq 1, >> length 1480 >> 20:44:33.943172 IP (tos 0x0, ttl 64, id 58746, offset 1480, flags >> [none], proto ICMP (1), length 48) >> 10.100.1.254 > 9.9.9.9: ip-proto-1 >> >> >> In the VM, the XDP program only sees the first packet, not the fragment. >> I added a printk to the program (see diff below): >> >> $ cat trace_pipe >> <idle>-0 [003] ..s2 254.436467: 0: packet length 1514 >> >> >> Anything come to mind in the virtio xdp implementation that affects >> fragment packets? I see this with both IPv4 and v6. > > Not yet. But we do turn of tap gso when virtio has XDP set, but it > shouldn't matter this case. > > Will try to see what's wrong. >
I added this to the command line for the NICs and it works: "mrg_rxbuf=off,guest_tso4=off,guest_tso6=off,guest_ecn=off,guest_ufo=off" XDP program sees the full size packet and the fragment. Fun fact: only adding mrg_rxbuf=off so that mergeable_rx_bufs is false but big_packets is true generates a panic when it receives large packets.
