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Re: [Xen-devel] A little more clarification

To: xen-devel@xxxxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] A little more clarification
From: Mark Williamson <Mark.Williamson@xxxxxxxxxxxx>
Date: Thu, 3 Mar 2005 15:33:49 +0000
Cc: Ted Hilts <thilts@xxxxxxxxxxxxxxxx>, Tim.Deegan@xxxxxxxxxxxx
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Hi Ted,

> Is what I have assumed correct, must there be an existing 2.4 or 2.6.x
> kernel based Linux installation up and running which must be modified
> (kernel modification or ported) to run Xen?


> It is my understanding that 
> the Xen source code installation or the binary installation both require
> a properly pre installed fully functional Linux system as the basis of
> Xen operation.  Once it's (pre installed linux distribution) 2.4.x or
> 2.6.x kernel has been modified to work with the Xen software and tools
> we have created this Linux distribution into an Xen-based system called
> Domain 0 that becomes so when shutdown and then booted???


> #1 -- The BUILD:
> The  Xen distribution includes 3 main components (Xen itself, ports of
> Linux 2.4 and 2.6.x kernels to run on (with) Xen, and user-space tools
> required to manage an Xen-based system) all of which assumes the
> existence of an already installed and fully fuctional  pre existing
> Linux system which must be adapted (to become the default Xen Linux
> build identified as DOMAIN 0.


> The Xen 
> process (called xend) runs in highest privilege in DOMAIN 0 and  it's
> code detects and starts secondary processors, sets up interrupt routing,
> time slicing, and performs PCI bus enumeration as well as offloads
> hardware support issues to DOMAIN 0 GUEST OS which is the modified pre
> existing Linux installed distribution.

You're misunderstanding here - the lowlevel stuff (secondary processors, 
interrupt routing, PCI, scheduling domains, etc, etc) happens in Xen.  Xen != 

Xen itself sits *below* the kernels of all the domains on the system 
(including domain 0) and handles the low-level details of the system.  The 
guest kernels are ported to run on top of Xen by using the interfaces it 

Xend is a management process that deals with the high-level management side of 
the system.  It builds domains, records what domains are running, sends them 
control messages, provides access to their consoles via TCP etc.  Xend isn't 
required for the system to run, just to perform these management-plane 

> As such, Xen provides a secure 
> virtual machine for this GUEST OS, builds other domains using an OS
> installed in a root file system placed on a partition and booted from
> that partition, manages their virtual devices and performs
> administrative tasks.

Root filesystems can be stored anywhere dom0 can access like a block device.  
Files, LVM volumes, MD devices, whole disk drives, partiitons, network block 
devices, etc. etc.

> My next question:
> What exactly is an HTTP/S server -- apparently it is a requirement for
> browser administration access to a Xen-based system? Is Apache such a
> server and if not can it be turned into such a server???

HTTPS?  It's a secured version of HTTP.  I don't think you shouldn't need to 
install anything extra to make this work - Twisted includes its own HTTP 

(btw, Apache can serve over HTTPS but we don't use it in Xen)

> One more question:
> Regarding the source code.  I have been reviewing Python and C
> (gcc/gcc++) and was wondering if there was a high level diagram showing
> dependencies (hierarchy based on caller and called)  and code utilized
> (python, C, binary insertions, etc.)???

Nope, not really :-)  The Xen User Manual, Xend manual and Xen Interface 
Manual (in the docs) directory might help you understand a bit more about the 
system.  To comprehend the source code itself, I'm afraid there's not really 
any kind of map.


> Based on what I am learning here (mostly from you) I have been
> constructing a very high level chart showing levels, key Xen processes,
> and stuff like that.  I will send you a copy once I am sure the
> information is correct and properly represents what is happening.
> Thank you very much in advance for your comments and advice.
> Ted

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