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[Xen-devel] pv_ops dom0 kernel and VT cpu extensions

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Subject: [Xen-devel] pv_ops dom0 kernel and VT cpu extensions
From: Gerry Reno <greno@xxxxxxxxxxx>
Date: Mon, 28 Dec 2009 23:53:20 -0500
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I started this discussion over on xen-users and I'm bringing it over to xen-devel.


I compiled a pv_ops dom0 kernel from Jeremy's latest git tree and upgraded Xen to 3.4.1 following one of Boris' blog entries. From the Xen perspective everything seems to work as expected. But, I want to be able to run both Xen and KVM existing guests in a single cloud on a farm of VT-enabled machines. So after booting into the new pv_ops dom0 kernel as a test I tried starting one of my KVM guests. It ran extremely slow so I knew immediately that it wasnt' using the VT acceleration. I then looked at /proc/cpuinfo and saw that the VT cpu flag (svm on this machine) was not there. The machine has a VT processor and hardware virtualization is enabled in the BIOS and I rebooted to check with a regular kernel and it of course the guest runs with VT acceleration there. My question is how can I get pv_ops dom0 kernel or Xen 3.4.1 to pass the cpu VT flags through so that existing KVM guests will run with cpu VT acceleration at full speed?

Fajar A. Nugraha wrote:
> On Tue, Dec 29, 2009 at 8:11 AM, Gerry Reno <greno@xxxxxxxxxxx> wrote:
> >> My question is how can I >> get pv_ops dom0 kernel or Xen 3.4.1 to pass the cpu VT flags through so that
>> existing KVM guests will run with cpu VT acceleration at full speed?
>> >
> AFAIK you can't. The same reason why you can't get Virtualbox and KVM
> to use VT together.
> Only one virtualization technology can use VT at the same time.
> As physical boxes gain more and more processing capability it makes no sense to restrict a physical machine to only a single hypervisor.

Libvirt will support nested VM's that pass through the VT capabilities to the next level. So it makes sense that Xen should be able to do the same thing and pass through the cpu VT capabilities to other hypervisors. Is there some law of the universe that prevents this?



Chris wrote:
> Following that logic we will need to start working on a vmmm (virtual machine monitor monitor) to handle multiple vmm's :)

.....What you say about monitor monitor is not quite what is needed but somewhat along those lines.

We need some type of small hypervisor-monitor / scheduler-kernel that would exist in Ring 0 and mediate between hypervisors. Then all hypervisors/kernels could be in Ring 1. Domains in Ring 2. All apps in Ring 3. Something like this.



So with libvirt being able to pass the cpu VT flags between nested levels can Xen do this as well and pass the cpu VT flags so that a KVM guest would be able to run with VT acceleration at full speed? How soon could this capability be added to pv_ops dom0 kernel or Xen?


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