Live migration between Intel Q6600 and AMD Phenom II

Live migration between Intel Q6600 and AMD Phenom II

[Sterling Windmill]

I've read that it's possible to live migrate KVM guests between Intel and AMD CPUs, is it also possible to migrate from a CPU without NPT/EPT to the Phenom II that supports NPT? Will I lose out on any of the benefits NPT allows without shutting down and restarting the guest?

Also, any thoughts on how much more performant a 3.0GHz Phenom II will be for running KVM guests than the 2.4GHz Intel Q6600?

Thanks in advance.

Re: Live migration between Intel Q6600 and AMD Phenom II

[Amit Shah]

Hello,

On (Tue) Sep 08 2009 [13:32:39], Sterling Windmill wrote:
> I've read that it's possible to live migrate KVM guests between Intel and AMD CPUs, is it also possible to migrate from a CPU without NPT/EPT to the Phenom II that supports NPT? Will I lose out on any of the benefits NPT allows without shutting down and restarting the guest?

Live migration between different vendors isn't tested enough for us to
be confident in saying it works well. There has been some work done in
the area, but there can always be bugs or unimplemented features. If you
do try it, please share your experiences whether good or bad.

I think NPT support should get enabled for your VM if the migration does
succeed.

> Also, any thoughts on how much more performant a 3.0GHz Phenom II will be for running KVM guests than the 2.4GHz Intel Q6600?

The Q6600 does not support EPT, right? If so, the Phenom will be faster
as it does support NPT.

		Amit

Re: Live migration between Intel Q6600 and AMD Phenom II

[Andre Przywara]

Sterling Windmill wrote:
> I've read that it's possible to live migrate KVM guests between Intel
 >  and AMD CPUs, is it also possible to migrate from a CPU without
 > NPT/EPT to the Phenom II that supports NPT?
Yes.
 > Will I lose out on
 > any of the benefits NPT allows without shutting down and restarting
 > the guest?
NPT/EPT is a hypervisor feature, so the guest is unaffected (left alone 
the performance benefit). The guest does it's own paging (the same as on 
native), so the actual hypervisor page handling is not visible to the 
guest. You will immediately benefit from NPT if you migrate from a 
non-EPT/NPT capable host.

Cross-vendor migration experience differs from guest to guest, the most 
important point to look at are the CPUID bits. Start with a conservative 
setting (like -cpu kvm64 if you have a recent qemu-kvm) and if in need 
turn on features that you need (which must be on both hosts).
Also use recent KVM kernel modules, some CVM related bugs have been 
fixed lately.
We have tried migrating several guests without issues between non-NPT 
and NPT hosts.
If you experience problem, feel free to mail me with a description.

> Also, any thoughts on how much more performant a 3.0GHz Phenom II will be for running KVM guests than the 2.4GHz Intel Q6600?
NPT is faster than shadow paging  on almost all workloads (since it 
avoids most of the #VMEXITs), but since the virtualization 
implementation differs in many details between Intel and AMD, 
performance estimation just based on this single feature is not very 
legitimate.

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448 3567 12
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