xdp and fragments with virtio

[David Ahern]

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.

Thanks,
David

[1] xdp program diff showing printk that dumps packet length:

diff --git a/samples/bpf/xdp_fwd_kern.c b/samples/bpf/xdp_fwd_kern.c
index 4a6be0f87505..f119b506e782 100644
--- a/samples/bpf/xdp_fwd_kern.c
+++ b/samples/bpf/xdp_fwd_kern.c
@@ -52,6 +52,11 @@ static __always_inline int xdp_fwd_flags(struct
xdp_md *ctx, u32 flags)
        u16 h_proto;
        u64 nh_off;

+       {
+               char fmt[] = "packet length %u\n";
+
+               bpf_trace_printk(fmt, sizeof(fmt), ctx->data_end-ctx->data);
+       }
        nh_off = sizeof(*eth);
        if (data + nh_off > data_end)
                return XDP_DROP;