Are you using the same binding as mentioned in previous mail sent by you? it
might be caused by cpu convention between pktgen and vhost, could you please
try to run pktgen from another idle cpu by adjusting the binding? 

I don't think that's the case -- I can cause pktgen to hang in the guest
without any cpu binding, and with vhost disabled even.

BTW, did you see any improvement when running pktgen from the host if no 
regression was found? Since this can be reproduced with only 1 vcpu for
guest, may you try this bind? This might help simplify the problem.
  vcpu0  -> cpu2
  vhost  -> cpu3
  pktgen -> cpu1 

Yes -- I ran the pktgen test from host to guest with the binding
described.  I see an approx 5% increase in throughput from 4.12->4.13.
Some numbers:

host-4.12: 1384486.2pps 663.8MB/sec
host-4.13: 1434598.6pps 688.2MB/sec