linux-kernel - Re: [PATCH] xen: core dom0 support

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Message-ID: <49AED391.1030500@zytor.com>
Date:	Wed, 04 Mar 2009 11:16:33 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen: core dom0 support

Anthony Liguori wrote:
> 
> I think this is a bit misleading.  I think you can understand the true 
> differences between Xen and KVM by s/hypervisor/Operating System/. 
> Fundamentally, a hypervisor is just an operating system that provides a 
> hardware-like interface to it's processes.
> 
[...]

> 
> The real difference between KVM and Xen is that Xen is a separate 
> Operating System dedicated to virtualization.  In many ways, it's a fork 
> of Linux since it uses quite a lot of Linux code.
> 
> The argument for Xen as a separate OS is no different than the argument 
> for a dedicated Real Time Operating System, a dedicated OS for embedded 
> systems, or a dedicated OS for a very large system.
> 

In particular, Xen is a microkernel-type operating system.  The dom0 
model is a classic single-server, in the style of Mach.  A lot of the 
"Xen could use a distributed dom0" arguments were also done with Mach 
("the real goal is a multi-server") but such a system never materialized 
(Hurd was supposed to be one.)  Building multiservers is *hard*, and 
building multiservers which don't suck is even harder.

	-hpa
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