CentOS virt - Nov 2010 - Is kvm-clock dependant on hardware of Physical Server?

I've got a three Centos 5.5 KVM severs, but only one of them is presenting
guests with the kvm-clock source.  

The two servers I'm not seeing kvm-clock on have Athlon x2 64bit
processors.   

The other server with kvm-clock has a Phenom II X6 1090T Processor. 
 
I've checked the system configuration and all the boxes have the same set
of RPM packages. To confirm that the issue was physical
hardware specific I migrated a VM from the Athlon to the Phenom server and
the kvm-clock source appeared.  

Any ideas why I'm not seeing kvm-clock with my Athlon X2 processors, or is
there another difference I haven't caught?
 

 Steve

On Thu, 04 Nov 2010 17:03:33 +1300, Steven Ellis
<mail_lists at stevencherie.net> wrote:
> 
> I've got a three Centos 5.5 KVM severs, but only one of them is
presenting
> guests with the kvm-clock source.
> 
> The two servers I'm not seeing kvm-clock on have Athlon x2 64bit
> processors.
> 
> The other server with kvm-clock has a Phenom II X6 1090T Processor.
> 
> I've checked the system configuration and all the boxes have the same
set
> of RPM packages. To confirm that the issue was physical
> hardware specific I migrated a VM from the Athlon to the Phenom server
and
> the kvm-clock source appeared.
> 
> Any ideas why I'm not seeing kvm-clock with my Athlon X2 processors, or
is
> there another difference I haven't caught?
> 
Well it looks like the issue relates to constant_tsc.

Out of the 3 boxes only the Phenom has constant_tsc in the CPU flags. There
appears to be some kernel list references here -
* http://www.mail-archive.com/kvm at vger.kernel.org/msg10193.html

This finally led to the following docs
 *
http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.5/html/Virtualization_Guide/chap-Virtualization-KVM_guest_timing_management.html

Looks like I need to do some tweaking at the grub level for my Athlon X2
boxes

Steve