Bug 705317

Summary:	Failed to ping guest(rhel4.9) after change virtio MTU
Product:	Red Hat Enterprise Linux 5	Reporter:	Golita Yue <gyue>
Component:	kvm	Assignee:	Michael S. Tsirkin <mst>
Status:	CLOSED WONTFIX	QA Contact:	Virtualization Bugs <virt-bugs>
Severity:	medium	Docs Contact:
Priority:	medium
Version:	5.7	CC:	gcosta, juzhang, mkenneth, virt-maint
Target Milestone:	rc
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	Bug Fix
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2011-05-18 13:28:51 UTC	Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Bug Depends On:
Bug Blocks:	580946

Description Golita Yue 2011-05-17 11:05:48 UTC

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

Comment 3 Glauber Costa 2011-05-18 13:28:51 UTC

This is due to lack of jumbo frame support in RHEL4.
Due to schedule constraints, we don't plan to fix it.

For more information, see 633195, opened against 6.1