[Fedora-xen] Fedora 7 kernels

[Daniel P. Berrange]

On Tue, Sep 25, 2007 at 04:17:24PM -0400, Jason Solan wrote:
> I've had a lot of problems with Fedora 7 and XEN and I've read some
> messages on this mailing list indicating the F7 kernel is more or less
> broke with xen.
> By problems I mean that 32 bit guests on 64 bit dom0 does not work at
> all for me.  Neither paravirtualized (as was a main feature of xen 3.1)
> or fully virtualized.

The release notes associated with Xen 3.1 turned out to be not quite
accurate on the subject of 32-on-64 & we didn't realize this when we
made corresponding release notes for Fedora 7. 

Xen 3.1 lets you run a 32-bit DomU kernel on a 64-bit hypervisor + Dom0
*ONLY IF* the DomU kernel was also based on Xen 3.1.  ie, a 32-bit DomU
kernel based on older Xen 3.0.* release would will not work on Xen 3.1
Unfortunately all the Fedora kernels for FC6 are 3.0.3 and  F7 GA was 
based on Xen 3.0.4, so neither will work on a Xen 3.1 hypervisor + Dom0.

In the current Xen-unstable upstream (which will become Xen 3.2) there
is now back-compat support for running older DomU kernels. In the most
recent update of Xen in  F-7 updates-testing we have included a handful
of backports which at least let us boot a 32-bit FC6 and F7  domU installer
kernel on a 64-bit F7 / F8 host.  

There are ongoing fixes in upstream Xen for various stability issues
when running 32-on-64 though, so it is not perfect. You may be lucky 
with our latest updates and have it work, but equally you may not be
lucky. We're tracking upstream & will pull in fixes when available.

> Combinations I've tried:
> F7 x86_64 dom0 -> F7 x86 domU PV

Works in basic testing. Has stability problems.

> F7 x86_64 dom0 -> F7 x86 domU HVM

F7 HVM has been broken since GA for reasons unrelated to the 32-on-64
stuff. A last minute clock-source & bootloader change screwed things
up. Yesterday we pushed newer kernel-xen and xen RPMs to updates-testing
which hopefully make this work.

> RHEL 5 x86_64 dom0 -> F7 x86 domU HVM

Not going to work - missing the fix I mention above.

> RHEL 5.1 Beta x86_64 dom0 -> F7 x86 domU PV

Works for me, at least to boot the installer. There are sometimes some
stability issues though, since Xen 3.1 was far from complete in its
32-on-64 support.

> RHEL 5.1 Beta x86_64 dom0 -> F7 x86 domU HVM

Not going to work - missing the fix I mention above.

> RHEL 5 x86 dom0 -> F7 x86 domU PV  -- Only one that works (32 on 32 PV)

Matched 32-on-32 and 64-on-64  kernels have always worked. 

> I had read a while ago (June?) that a kernel update would be coming soon
> to fix the 32 on 64 PV issue, however I've ran in several new kernels
> and none seem to fix the issue.
> https://www.redhat.com/archives/fedora-xen/2007-June/msg00031.html

The fix required both kernel-xen & xen updates which are finally available
in updates-testing.

> I've also repeatedly tried to run a 32 bit HVM fedora 7 guest on 64 bit
> machines and had that fail similar to this:

F7  HVM guests have known issues, also hopefully now fixed.

> http://www.redhat.com/archives/fedora-xen/2007-June/msg00094.html
> 
> Is there any more work planned for Fedora 7 and xen, or should I just
> wait until F8?  Or am I just doing something wrong and the problems I'm
> having should be working on the current builds?


Fedora 7 will continue to have updates until its end-of-life. The issue
is that even in upstream Xen 32-on-64 doesn't have the same level of 
stability as 32-on-32, or 64-on-64.

Dan.
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