KVM & Xen coexist?

KVM & Xen coexist?

[Jun Koi]

hi,

i guess this question should be in your FAQ: does KVM work with Xen?
in the other word, is it possible to run KVM on Xen machine? (i.e
loading KVM on Dom0 or DomU. obviously it does not work on DomU, but i
am not sure about Dom0 case).

cheers,
J

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Re: KVM & Xen coexist?

[Avi Kivity]

Jun Koi wrote:
> hi,
>
> i guess this question should be in your FAQ: does KVM work with Xen?
> in the other word, is it possible to run KVM on Xen machine? (i.e
> loading KVM on Dom0 or DomU. obviously it does not work on DomU, but i
> am not sure about Dom0 case).
>   

Theoretically, you can run Xen for paravirtualization and kvm for full 
virtualization.  Realistically, that's not trivial to accomplish:

- Xen adds an additional addressing abstraction (machine addresses) that 
kvm has to be taught about
- kvm uses privileged instructions which cannot be executed by a Xen 
guest (and would thus need assists)
- VT returns to ring 0, and would need an assist to return to ring 1 
where Linux (and kvm) lives
- kvm wants to be pinned to a cpu while it's modifying VT state

Another option is to use a small subset of kvm to run paravirtualized 
Xen guests on top of kvm (without Xen itself).

-- 
error compiling committee.c: too many arguments to function


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Re: KVM & Xen coexist?

[Arnd Bergmann]

On Wednesday 08 November 2006 10:32, Avi Kivity wrote:
> Another option is to use a small subset of kvm to run paravirtualized 
> Xen guests on top of kvm (without Xen itself).

How about running Xen inside of kvm? Since kvm can do full
virtualization, it should work for all versions of Xen that
don't use the HV instruction set extension themselves, right?

	Arnd <><

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Re: KVM & Xen coexist?

[Muli Ben-Yehuda]

On Wed, Nov 08, 2006 at 02:43:21PM +0100, Arnd Bergmann wrote:
> On Wednesday 08 November 2006 10:32, Avi Kivity wrote:
> > Another option is to use a small subset of kvm to run paravirtualized 
> > Xen guests on top of kvm (without Xen itself).
> 
> How about running Xen inside of kvm? Since kvm can do full
> virtualization, it should work for all versions of Xen that
> don't use the HV instruction set extension themselves, right?

Yes. I'm not quite sure why you would want to do this though, except
maybe because it's possible and thus someone, somewhere wants to see
it done.

Cheers,
Muli

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Re: KVM & Xen coexist?

[Arnd Bergmann]

On Wednesday 08 November 2006 14:46, Muli Ben-Yehuda wrote:
> Yes. I'm not quite sure why you would want to do this though, except
> maybe because it's possible and thus someone, somewhere wants to see
> it done.

I don't know what tools exist for debugging xen itself, but I could
imagine that it's rather helpful if you can just attach gdb from
Linux to your running KVM guest to single-step through the xen
code. Other than that, there's probably not much point.

	Arnd <><

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Re: KVM & Xen coexist?

[Anthony Liguori]

Avi Kivity wrote:
> Jun Koi wrote:
>> hi,
>>
>> i guess this question should be in your FAQ: does KVM work with Xen?
>> in the other word, is it possible to run KVM on Xen machine? (i.e
>> loading KVM on Dom0 or DomU. obviously it does not work on DomU, but i
>> am not sure about Dom0 case).
>>   
> 
> Theoretically, you can run Xen for paravirtualization and kvm for full 
> virtualization.  Realistically, that's not trivial to accomplish:

It would be much more reasonable to do paravirtualization under Linux 
with something like lhype.  There was an announcement (although I never 
saw code) of a kernel module that could run Xen guests.

For all practical purposes, Xen's HVM support is already KVM under Xen. 
  Of course the Xen "version" of KVM has many more QEMU patches and has 
a lot of other fun stuff (like Xend/Xenstore--ugh).

> - Xen adds an additional addressing abstraction (machine addresses) that 
> kvm has to be taught about
> - kvm uses privileged instructions which cannot be executed by a Xen 
> guest (and would thus need assists)
> - VT returns to ring 0, and would need an assist to return to ring 1 
> where Linux (and kvm) lives
> - kvm wants to be pinned to a cpu while it's modifying VT state
> 
> Another option is to use a small subset of kvm to run paravirtualized 
> Xen guests on top of kvm (without Xen itself).

It would probably be more useful to run VMI guests (or to implement a 
set of paravirt_ops just for KVM).  The Xen guest ABI is awfully 
complicated.  The drivers would require a ton of userspace infrastructure.

Regards,

Anthony Liguori

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