Description of problem:
I booted up a rhel4.9 guest with virtio nic, change the MTU of guest nic and tap device to 65520 (max MTU of virtio nic), and added an arp entry on host, then ping guest from host, but it was failed.

Version-Release number of selected component (if applicable):
host:
kvm-83-232.el5
kernel 2.6.18-259.el5

guest(rhel4.9):
kernel 2.6.9-100.ELhugemem 
this problem happened in rhel4.9-32/64/PAE 

How reproducible:
100%

Steps to Reproduce:
1. boot guest with virtio nic by cmd

2. Change MTU of guest nic and $tap
guest)# ifconfig eth0 mtu 65520
host)# ifconfig $tap mtu 65520

3. Add a arp entry on host
# arp -s $guest_ip $guest_mac -i $tap

4. Ping guest from host  (65520-28=65492, 65520 is max mtu of virtio)
#ping $guest_ip -I $tap -s 65492 -M do
  
Actual results:
could not ping guest successfully

Expected results:
can ping guest successfully

Additional info:
cmd:
/usr/libexec/qemu-kvm -drive file='RHEL-4.9-PAE-virtio.raw',index=0,if=ide,media=disk,cache=none,format=raw -net nic,vlan=0,model=virtio,macaddr='9a:85:ef:26:a1:34' -net tap,vlan=0,ifname='t0-134629-2A7D',script='/etc/qemu-ifup-switch',downscript='no' -m 4096 -smp 
2,cores=1,threads=1,sockets=2 -cpu qemu64,+sse2 -soundhw ac97 -vnc :0 -rtc-td-hack -M rhel5.6.0 -boot c  -usbdevice tablet -no-kvm-pit-reinjection