Bug 481559

Summary:	jumbo frames (> 4064-byte MTU) fail in Xen domU
Product:	Red Hat Enterprise Linux 5	Reporter:	Matt Bernstein <mb--redhat>
Component:	kernel-xen	Assignee:	Xen Maintainance List <xen-maint>
Status:	CLOSED CURRENTRELEASE	QA Contact:	Red Hat Kernel QE team <kernel-qe>
Severity:	medium	Docs Contact:
Priority:	low
Version:	5.3	CC:	kernel-maint, quintela, xen-maint
Target Milestone:	rc
Target Release:	---
Hardware:	x86_64
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	Bug Fix
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2009-04-16 22:45:54 UTC	Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:
Bug Blocks:	492570

Description Matt Bernstein 2009-01-26 11:26:03 UTC

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 kernels (CentOS 5.2 and Fedora 8).

I don't know about F9, but the pv_ops unified F10 kernel will work if I configure the MTU on eth0 <= 4064 bytes, but at >= 4065 bytes large packets seem to be silently dropped somewhere.

"yum clean all; yum upgrade" just stiffs until I lower the MTU.

Only tried on x86_64.

Comment 1 Matt Bernstein 2009-02-20 13:13:53 UTC

F9 has the same problem.

Comment 2 Matt Bernstein 2009-04-13 10:45:22 UTC

I noticed that the number changed from 4064 to 4048 (another 32 bytes below 4K), on a domU kernel upgrade / xen dom0 upgrade and it looks like the problem is dom0-specific rather than domU-specific.

It's 4064 bytes for dom0 2.6.18-92.1.18.el5xen / xen-3.0.3-73.

It's 4048 bytes for dom0 2.6.18-92.1.22.el5xen / xen-3.0.3-80.el5.

Both are CentOS 5.2 kernels (5.3 crashes, but that's a different issue, I think); the first xen is a backport from RHEL5.3 beta, the second xen a plain CentOS 5.3.

So, I don't know if this is a hypervisor kernel or Xen bug (or neither, given it only appears to affect pv_ops domU kernels), so I thought I'd change the component to Xen in case it's interesting to different pairs of eyes.

Comment 4 Bill Burns 2009-04-15 11:45:59 UTC

Please let us know if it fails on RHEL 5.3.

Comment 5 Matt Bernstein 2009-04-16 21:57:03 UTC

OK, I can report success on dom0 CentOS 5.3 kernel-xen-2.6.18-128.1.6.el5 / xen-3.0.3-80.el5_3.2 / domU Fedora 10 2.6.29.1-15.fc10.x86_64 / 9000-byte frames.

So I still don't know what the bug was, but am happy RHEL5.3 fixes it.

Thanks; as far as I'm concerned you can close this bug.

Comment 6 Bill Burns 2009-04-16 22:45:54 UTC

Thanks for the testing.