Re: [Xen-users] Xen - Operating System or Application

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Christopher McCormick wrote:

> Understanding that "The Xen hypervisor is a layer of software 
> running directly on computer hardware replacing the operating system 
> thereby allowing the computer hardware to run multiple guest 
> operating systems concurrently." is Xen an operating system in the 
> true sense of the term or is it an application that provides 
> operating system like functionality?
Neither !
It is a hypervisor. it sits between the hardware and the OSs.

If you look from the viewpoint of the hardware then it replaces what 
you would conventionally consider to be the OS. When you boot, Xen is 
what gets loaded - in place of Windows, Linux, BSD, or whatever your 
would otherwise have been running.
Xen then loads and runs one or more OSs (even Dom0 is a guest to Xen).


If you look from the other end, then from the OS POV, Xen has 
replaced the bare metal hardware. So for each guest, access to the 
hardware is now mediated by this new layer.

So if you are used to an "onion ring" diagram with hardware in the 
middle, and layers labelled "kernel, OS, utilities, applications" as 
you go out, then you now have a new layer so the rings now read 
"hypervisor, kernels, OSs, utilities, applications" - with the key 
difference that now you would have "pie slices" dividing up all but 
the hypervisor rings into segments for each guest.

In Xen, Dom0 is still a guest - but it has very privileged access to 
any hardware that hasn't been hidden from it. This privileged access 
does give the appearance that it's running directly on the hardware 
but as I understand it that isn't the case. Also, each guest runs as 
a separate VM controlled/managed by Xen - and at the same level 
(distance out in the onion rings) as Dom0.
Other virtualisation systems work differently. Some of them start the 
privileged OS on the bare metal, and then guests are a guest of this 
parent OS. Thus your onion rings start with "privileged domain plus 
hypervisor" in the inner ring, and then the guest OSs in a layer 
outside of that.
Of course, it's not as simple as there being distinct layers. Xen 
won't do a great deal for you without active support from tools 
running in Dom0 - so in a way the line between hypervisor and 
privileged domain is a bit blurred. Ie, it;s not so different from 
the old argument about the lines in a typical "Linux" system are 
between kernel, OS, and utilities.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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