On Mon, Jun 3, 2019 at 02:53 AM, Toshiaki Makita wrote:

You should not need SKB mode on kernel 5.0.
Do you attach any XDP program on vbox1 and vbox2? If not, redirected packets will be dropped.
Please refer to the slides below for details.
https://netdevconf.org/0x13/session.html?talk-veth-xdp

Thanks for your material. It is really helpful. 
After attach XDP_PASS program on vbox1 and vbox2, and set the right dst mac address in xdp redirect program, I can now ping success from ns1 to ns2 (192.168.1.2->192.168.2.2) :)