[Gllug] Xen - bit of a ramble

[Ryland, Peter]

On Tue, 2006-05-16 at 02:26 -0700, Rob Bannocks wrote:
> While we are on the topic, can someone explain
> to me concisely the diffrence between xen and 
> usermode linux?

Usermode linux is basically Linux ported to run as a process under
linux.  It doesn't require privileged access to the hardware, but
instead forwards system calls on to the host kernel.  Therefore, just as
you can have two users on a system fight for resources, you can have two
UML instances fighting in the same way, with limited ability of the
admin to control it.  In terms of isolation of virtual machines, this
isn't terribly good.

Ok, so we all know about how the 80386 revolutionised intel's line of
CPUs by adding memory protection and IO protection and such.  How it
achieves this is by enabling processes to run in different "rings".  If,
say, a process in ring 3 does some IO which it's not allowed to do, or
requests a page of memory that is swapped out or something, a parent
process in a higher ring, say ring 0, can get word of this before it's
carried out and can veto the action (causing a segfault), or swap the
page in or whatever it deems appropriate.  So normal linux uses this on
x86 to protect programs from each other and the system from programs by
running the kernel in ring 0 with full access to everything and user
programs in ring 3 with only the ability to make system calls to the
kernel.

The idea behind Xen is to run a "monitor" kernel in ring 0 which acts as
a scheduler for the virtual machine kernels which have been ported to
run under this and in ring 1, and then have user processes unchanged
running in ring 3 as normal.  This allows it to have fairly good
isolation between VMs and in practice very minimal overhead.  In fact, a
ring 1 kernel can even be given actual hardware access for a piece of
hardware to enable it to use native drivers for that hardware,
eliminating any overhead at all.

HTH,
Pete


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