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Re: [Xen-users] Prohibit to run Xen inside guest VM?

On Tue, Dec 15, 2009 at 10:30 AM, Jun Koi <junkoi2004@xxxxxxxxx> wrote:
> On Tue, Dec 15, 2009 at 11:24 AM, Fajar A. Nugraha <fajar@xxxxxxxxx> wrote:
>> On Tue, Dec 15, 2009 at 9:09 AM, Jun Koi <junkoi2004@xxxxxxxxx> wrote:
>>> Hi,
>>> I am looking around to find an ISP that provides Xen VM, so I can run
>>> whatever OS I want inside the rented guest VM.
>> usually you don't get to run "whatever OS". Amazon EC2 is a good
>> example. Look at what OS they support.
> The point is: once I control my VM, I can replace the OS anytime, no?
> How can they enforce that?
> (Suppose that they give me the HVM)

That depends. Do they allow you to change domU's disks? Do they allow
attaching ISO image? Do they give vnc console access? If all of those
is yes, then you should be able to replace it. AFAIK they usually
start with a prebuilt image.

>> Technically the only way you could run nested Xen is by using a HVM
>> domU. Even then you'd get close to dead snail performance, and you can
>> only have PV guests.
> Why we can only have PV guests, but not HVM?
> I know that currently nested hypervisor is not good in performance,
> but that might very well change in the future.

HVM requires a special CPU feature (AMD-V or intel-VT) that can only
be used by the hypervisor, and not by domU. Which is why you can't
have nested HVM guest or (in the case of Windows) nested Hyper-V

This is different from Vmware or Virtualbox, which can work (for 32bit
guest OS anyway) even without the CPU feature.


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