[Gllug] performance of xen dom0 vs native linux

[Nahuel Marisi]

Thanks Bruce for the  detailed explanation. I have seen various diagrams
explaining some of the concepts you've highlighted here.

Teh only thing I don't quite get is how the hypervisor controls the hardware
by using dom0's kernel modules.

2009/5/8 Bruce Richardson <itsbruce at workshy.org>

> 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/Ring_%28computer_security%29>
> 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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