Re: [Xen-users] Basic questions - numbered

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

Some snippets answered, hopefully others will fill in the rest.

>  2. Machine application processing architecture.

Once you have got a SuSE 9.x install onto the drive, there is a YaST tool for 
"install into a directory" you can use to create other domain filesystems.

> The Xen maual indicates we should simply install additional OS systems
> from the disk(s) that make up the distribution.

Take a look at the appendix on using debbootstrap for an easy way to install 
Debian.

> short experience with the LIVE CD Xen seems to be always running and the
> DOM 0 Guest (Debian in the case of the LIVE CD) can be started and
> stopped.  I had previously thought that DOM 0 Guest might be an
> exception -- always virtualized?

Dom0 also runs on top of Xen, as a guest (albeit with extra privileges to 
access hardware).

> It took me a while to understand the following -- I hope it's right???
>  If it's wrong even in the slightest will someone please correct it.
>
>  From a high level perspective, "Virtualization by Xen" is the
> controlled execution of kernels associated with physical installations
> of i86 based OSs.  But it is more. Xen is integrated with the modified
> kernel of a Linux i86 based OS installation -- the base installation
> (and therefore the physical resources hard drive, NIC, etc. of that base
> installation) which combination is called DOM 0.

That's about right.  Xen controls access to CPU, memory, IO ports and 
interrupt lines.  That's it, it doesn't know or care about devices.

By talking to Xen's interface, privileged domains can talk to the actual 
hardware.  The first domain to start on the machine (dom0) is allowed to 
access all the hardware, using its own device drivers.

> Xen is responsible for 
> the sharing of these physical resources (associated with DOM 0) by means
> of time sliced (relatively concurrent) execution control of other
> installed Operating Systems

Xen shares out the resources listed above.  However, since dom0 controls the 
actual devices, it is responsible for sharing out access to block, network, 
etc. to other domains.

> called DOMU for user level (because they do 
> not have certain privileges and permissions and when  needed Xen manages
> that need).

Strictly I think the U is for "unprivileged" - domain kernels don't 
technically run in "user mode".  It doesn't really matter, the terminology 
evolves over time ;-)

> Xen provides interfaces and processes to accomplish this 
> controlled execution of DOMU types of domains or even other Xen-based
> DOM 0 domains where a domain is a primary or logical partition having a
> file sytem (formatted as ext2, ext3 or other) and a OS  (distribution)
> as physical content.

Domains can also run out of files in dom0's filesystem, LVM volumes in dom0, 
network block devices in dom0...

Anything dom0 can see as a block device can be used as a domU's root 
filesystem.

HTH,
Mark

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