[Xen-users] Re: [Xen-devel] DomU as Dom0?

[Steven Maresca <steve.maresca@xxxxxxxxx>]

> Date: Fri, 12 Feb 2010 13:06:25 -0200
> From: Thiago Camargo Martins Cordeiro <thiagocmartinsc@xxxxxxxxx>
> Subject: Re: [Xen-devel] DomU as Dom0?
> To: Sassy Natan <sassyn@xxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Xen List
>        <xen-users@xxxxxxxxxxxxxxxxxxx>
> Message-ID:
>        <6b7f6eb1002120706me206d81p5b211011dd21909d@xxxxxxxxxxxxxx>
> Content-Type: text/plain; charset="iso-8859-1"
>
> You can run the Xen, the dom0 and some domUs whitin a KVM guest...
>
> On 11 February 2010 17:43, Sassy Natan <sassyn@xxxxxxxxx> wrote:
>
>> Hi,
>>
>>
>> Can I run on DomU a Dom0 Kernel? So that This DomU will act as a Dom0 for
>> other DomU on top of it?
>>
>>
>>
>> Thanks
>>
>> Sassy

Yes, you can run Xen in a domU.

However, as usual the devil is in the details. Just to clarify
(because I don't believe it has been said explicitly):

It is entirely possible right now to run Xen + dom0 within an HVM
domU.  CPU support for hardware virtualization is a requirement. In
truth, this has been the case for quite some, since the Xen 3.0
series, when HVM support started to firm up around 3.0.4.   This mode
of operation presents some challenges due the hardware emulated by
qemu-dm, time skew, etc., but it has improved greatly with the latest
Xen releases.

This is the general idea of encapsulation:
Xen { dom0, domU { Xen { dom0, domU, .. } }, ... }

In general, this technique is useful for driver/hypervisor
development, but it presents some limitations:
-The nested Xen + dom0 can only support paravirtual guests.
-Finicky dom0 kernels require boot-time tweaking (I can dig up my
notes on the details if anyone is interested).
-Paravirtual I/O from the nested dom0 to the "parent" dom0 is
possible, but requires ugly driver hackery.
-Usually slow.  Very recent AMD/Intel processors, hardware assisted
paging, etc reduce the pain, but certainly not completely.
-Not what I would consider a production-ready/supportable environment.
Good for development/experimentation/education only.  There are
quirks..

The above largely applies when running Xen+dom0+domUs within a vm on
VMware / KVM / etc.

All of that said, the nested virtualization capabilities present in
KVM, those in the Xen pipeline (discussed in the summit presentation
previously mentioned), VMware, and others do open the door to
improvement in this area.

Ultimately, I would suggest that interested parties give this a try.
If needs are met, then it may be an entirely sufficient solution.  At
the very least, taking a trip down this particular rabbit hole will be
informative :)

Steve Maresca

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