hi,

i have found that guest and host can't communicate with each other with 
pcnet patch, but ne2000 is okey.
(http://dad-answers.com/qemu/patches/AMD-PCNET-II/qemu-pcnet.patch6.gz)

reproduce steps --
use attached qemu-ifup and do a "ifconfig eth0 172.20.0.2 up" in guest 
after boot.  the ping between guest and host doesn't  work.

root cause is that arp packet from tun interface (42 bytes) is small 
than the minimal size(60 bytes) and is droped by guest  pcnet driver.

attached patch(borrowed from ne2000) just expend the packet when size < 60.

any comments?

thanks,
edwin