bpf_debug output

Hi! 

I am working on some minor modifications in xdp-cpumap-tc to accommodate NAT at WAN side.
But the things are not working as expected.

How can bpf_debug be used to help debugging?

I read that its output ends-up in /sys/kernel/debug/tracing/trace_pipe.
But how can it be read? 
I tried "cat trace_pipe" but this produced no output whatsoever.

Any hints will be appreciated.

Thanx

Regards

Ethy