Recursive virtualization

Recursive virtualization

[Casey Jeffery]

I swear this has been brought up before in this forum, but I can't
find it. I'm curious what the virtualization gurus in this forum think
of the possibilities for recursive virtualization. I know vbox claims
to support it, but I haven't come across many details on how they do
it and I don't think they really use the hvm hardware. Is it something
that should be possible without an "enlightened" guest hypervisor and
by basically just virtualizing the VMCS/VMCB structures?

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Re: Recursive virtualization

[Dor Laor]

>I swear this has been brought up before in this forum, but I can't
>find it. I'm curious what the virtualization gurus in this forum think
>of the possibilities for recursive virtualization. I know vbox claims
>to support it, but I haven't come across many details on how they do
>it and I don't think they really use the hvm hardware. Is it something
>that should be possible without an "enlightened" guest hypervisor and
>by basically just virtualizing the VMCS/VMCB structures?

We have an open todo task for it.
It is 'just' emulating the VMCS/VMCB structures and commands.

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Re: Recursive virtualization

[Gregory Haskins]

It seems from cursory inspection that this is possible in theory, even on HVM hardware.  My thoughts are as follows (Intel oriented, which I know better):

*) The hypervisor sets to trap on VMX type operations (VMXON/OFF/START/RESUME, etc) and provide emulation of them as follows:

*) When a VMXON instruction is encountered, mark the guest as a nested hypervisor and setup any necessary structures for tracking

*) when a VMXSTART/RESUME instruction is encountered, launch the new guest as a subordinate guest of the owning hypervisor guest.  When a VMEXIT occurs, VMXRESUME the hypervisor guest.

etc etc

This in theory could work to any degree of nesting, as long as the VMX operations are emulated.  Note that each subordinate hypervisor would think it was emulating the VMX operations too, even though it is really only the bottom hypervisor which does so.  This is kind of mind bending ;)

-Greg


>>> On Wed, Apr 4, 2007 at 12:36 PM, in message
<cb6aceaa0704040936j62354e99vb49108040ff79d4e-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>, "Casey Jeffery"
<kcjeffery-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote: 
> I swear this has been brought up before in this forum, but I can't
> find it. I'm curious what the virtualization gurus in this forum think
> of the possibilities for recursive virtualization. I know vbox claims
> to support it, but I haven't come across many details on how they do
> it and I don't think they really use the hvm hardware. Is it something
> that should be possible without an "enlightened" guest hypervisor and
> by basically just virtualizing the VMCS/VMCB structures?
> 
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Re: Recursive virtualization

[Casey Jeffery]

Dor,

Thanks, I realize there will certainly be a lot of work in
virtualizing them. Maybe Intel can help out with VVT-x to give a
root-root mode. ;)

Any idea at a high level how vbox does it? I will post in their forum,
but I assume somebody here has a good idea.

Thanks.

On 4/4/07, Dor Laor <dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
> >I swear this has been brought up before in this forum, but I can't
> >find it. I'm curious what the virtualization gurus in this forum think
> >of the possibilities for recursive virtualization. I know vbox claims
> >to support it, but I haven't come across many details on how they do
> >it and I don't think they really use the hvm hardware. Is it something
> >that should be possible without an "enlightened" guest hypervisor and
> >by basically just virtualizing the VMCS/VMCB structures?
>
> We have an open todo task for it.
> It is 'just' emulating the VMCS/VMCB structures and commands.
>
> >
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Re: Recursive virtualization

[Dor Laor]

>
>Dor,
>
>Thanks, I realize there will certainly be a lot of work in
>virtualizing them. Maybe Intel can help out with VVT-x to give a
>root-root mode. ;)
>
>Any idea at a high level how vbox does it? I will post in their forum,
>but I assume somebody here has a good idea.


Vbox branched out from qemu. It is another emulator that suppose to run
in native code in a similar fashion to kqeumu. They run the user code
and non-dangerous stuff natively, the rest is fully emulated, thus can
have nested levels.

>
>Thanks.
>
>On 4/4/07, Dor Laor <dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>> >I swear this has been brought up before in this forum, but I can't
>> >find it. I'm curious what the virtualization gurus in this forum
think
>> >of the possibilities for recursive virtualization. I know vbox
claims
>> >to support it, but I haven't come across many details on how they do
>> >it and I don't think they really use the hvm hardware. Is it
something
>> >that should be possible without an "enlightened" guest hypervisor
and
>> >by basically just virtualizing the VMCS/VMCB structures?
>>
>> We have an open todo task for it.
>> It is 'just' emulating the VMCS/VMCB structures and commands.
>>
>> >
>>
>-----------------------------------------------------------------------
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share
>> >your
>> >opinions on IT & business topics through brief surveys-and earn cash
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>> EV
>> >_______________________________________________
>> >kvm-devel mailing list
>> >kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org
>> >https://lists.sourceforge.net/lists/listinfo/kvm-devel
>>

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