Bug 12580

Summary:	jumbo frames (> 4064-byte MTU) fail in Xen domU
Product:	Virtualization	Reporter:	Matt Bernstein (mb--kernelbug)
Component:	Xen	Assignee:	virtualization_xen
Status:	RESOLVED OBSOLETE
Severity:	normal	CC:	alan
Priority:	P1
Hardware:	All
OS:	Linux
Kernel Version:	2.6.27.7-134.fc10.x86_64	Subsystem:
Regression:	No	Bisected commit-id:

Description Matt Bernstein 2009-01-30 00:50:22 UTC

Latest working kernel version: 2.6.21.7-5.fc8xen.x86_64
Earliest failing kernel version: 2.6.27.7-134.fc10.x86_64 (didn't yet try F9)
Distribution: Fedora
Hardware/Software Environment: Xen host is CentOS 5.2

Problem Description:

With Xen dom0 physical, bridge and vif interface all set to 9000-byte MTUs, I
expect to be able to set the domU eth0 MTU to 9000-bytes and enjoy a happy
jumbo-frame life.

This works on the old-style Xen guest kernels (CentOS 5.2 and Fedora 8).

I don't know about Fedora 9, but the pv_ops unified Fedora 10 kernel will work if I configure the MTU on eth0 <= 4064 bytes, but at >= 4065 bytes large packets
seem to be silently dropped somewhere.

Steps to reproduce:

"yum clean all; yum upgrade" will provoke a large rx frame--it just stiffs until I lower the MTU.

Only tried on x86_64.

Comment 1 Matt Bernstein 2009-02-20 05:15:51 UTC

Earliest failing kernel version: 2.6.25.3-2.fc9.x86_64.xen (F9 has same bug)

Comment 2 Alan 2012-10-30 15:50:09 UTC

If this is still seen on modern kernels then please re-open/update