[Gllug] performance of xen dom0 vs native linux

[Bruce Richardson]

On Fri, May 08, 2009 at 10:41:42AM +0100, Nahuel wrote:
> 
> I was wondering what's the difference in performance between linux running
> in dom0 and no VMs (domUs) running compared to native linux.
> Obviously if you use something like virtualbox when there are no VMs running
> you just have linux running natively so you have no performance penalties.
> Is it the same if you're running Xen with no hosts?

No.  Virtualbox, like User-mode-Linux, is a process that is run by the
Linux kernel; stop the process and virtualition goes away.  The host
Linux kernel is running at the most privileged level and grants
privileges to the virtualbox guests.  With Xen, it's the Xen hypervisor
that runs first and it then runs (and grants privileges to) the Linux
Dom0.

Another way of saying this is that on a normal Linux system, the kernel
runs at Ring 0 and has complete control of the PC's archtecture, whereas
under Xen the hypervisor is at Ring 0 and the guest domains are at ring
1 or 3 and are presented with a partial view of the computer's resources
that is controlled by the hypervisor.

The Xen hypervisor uses the Dom0's kernel modules to control the PC
hardware, but it's the hypervisor that is in charge.  The Dom0 is *not*
the same as a regular Linux kernel; it has to be specially compiled with
Xen as a target.

> 
> Does it affect the way linux runs in any way?

It's a complex subject.  Here are some potentially useful links:

http://en.wikipedia.org/wiki/Ring_(computer_security)
http://en.wikipedia.org/wiki/Xen
http://www.dell.com/downloads/global/power/ps3q05-20050191-Abels.pdf

-- 
Bruce

Explota!: miles de lemmings no pueden estar equivocados.
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