--- wikisrc/ports/xen/howto.mdwn	2014/12/23 23:25:57	1.11
+++ wikisrc/ports/xen/howto.mdwn	2014/12/23 23:37:56	1.12
@@ -4,37 +4,51 @@ Introduction
 [![[Xen
 screenshot]](http://www.netbsd.org/gallery/in-Action/hubertf-xens.png)](../../gallery/in-Action/hubertf-xen.png)
 
-Xen is a virtual machine monitor for x86 hardware (requires i686-class
-CPUs), which supports running multiple guest operating systems on a
-single machine. Guest OSes (also called “domains”) require a modified
-kernel which supports Xen hypercalls in replacement to access to the
-physical hardware. At boot, the Xen kernel (also known as the Xen
-hypervisor) is loaded (via the bootloader) along with the guest kernel
-for the first domain (called *domain0*). The Xen kernel has to be loaded
-using the multiboot protocol. You would use the NetBSD boot loader for
-this, or alternatively the `grub` boot loader (`grub` has some
-limitations, detailed below). *domain0* has special privileges to access
-the physical hardware (PCI and ISA devices), administrate other domains
-and provide virtual devices (disks and network) to other domains that
-lack those privileges. For more details, see [](http://www.xen.org/).
-
-NetBSD can be used for both *domain0 (Dom0)* and further, unprivileged
-(DomU) domains. (Actually there can be multiple privileged domains
-accessing different parts of the hardware, all providing virtual devices
-to unprivileged domains. We will only talk about the case of a single
-privileged domain, *domain0*). *domain0* will see physical devices much
-like a regular i386 or amd64 kernel, and will own the physical console
-(VGA or serial). Unprivileged domains will only see a character-only
-virtual console, virtual disks (`xbd`) and virtual network interfaces
-(`xennet`) provided by a privileged domain (usually *domain0*). xbd
-devices are connected to a block device (i.e., a partition of a disk,
-raid, ccd, ... device) in the privileged domain. xennet devices are
-connected to virtual devices in the privileged domain, named
-xvif\<domain number\>.\<if number for this domain\>, e.g., xvif1.0. Both
-xennet and xvif devices are seen as regular Ethernet devices (they can
-be seen as a crossover cable between 2 PCs) and can be assigned
-addresses (and be routed or NATed, filtered using IPF, etc ...) or be
-added as part of a bridge.
+Xen is a virtual machine monitor or hypervisor for x86 hardware
+(i686-class or higher), which supports running multiple guest
+operating systems on a single physical machine.  With Xen, one uses
+the Xen kernel to control the CPU, memory and console, a dom0
+operating system which mediates access to other hardware (e.g., disks,
+network, USB), and one or more domU operating systems which operate in
+an unprivileged virtualized environment.  IO requests from the domU
+systems are forwarded by the hypervisor (Xen) to the dom0 to be
+fulfilled.
+
+Xen supports two styles of guests.  The original is Para-Virtualized
+(PV) which means that the guest OS does not attempt to access hardware
+directly, but instead makes hypercalls to the hypervisor.  This is
+analogous to a user-space program making system calls.  (The dom0
+operating system uses PV calls for some functions, such as updating
+memory mapping page tables, but has direct hardware access for disk
+and network.)   PV guests must be specifically coded for Xen.
+
+The more recent style is HVM, which means that the guest does not have
+code for Xen and need not be aware that it is running under Xen.
+Attempts to access hardware registers are trapped and emulated.  This
+style is less efficient but can run unmodified guests.
+
+At boot, the dom0 kernel is loaded as module with Xen as the kernel.
+The dom0 can start one or more domUs.  (Booting is explained in detail
+in the dom0 section.)
+
+NetBSD supports Xen in that it can serve as dom0, be used as a domU,
+and that Xen kernels and tools are available in pkgsrc.  This HOWTO
+attempts to address both the case of running a NetBSD dom0 on hardware
+and running NetBSD as a domU in a VPS.
+
+Prerequisites
+=============
+
+Installing NetBSD/Xen is not extremely difficult, but it is more
+complex than a normal installation of NetBSD.
+
+This HOWTO presumes a basic familiarity with the Xen system
+architecture.
+
+This HOWTO presumes familiarity with installing NetBSD on i386/amd64
+hardware and installing software from pkgsrc.
+
+For more details on Xen, see [](http://www.xen.org/).
 
 Installing NetBSD as privileged domain (Dom0)
 ---------------------------------------------
@@ -336,7 +350,7 @@ working vif-bridge is also provided with
 
     #!/bin/sh
     #============================================================================
-    # $NetBSD: howto.mdwn,v 1.10 2014/12/23 23:08:37 gdt Exp $
+    # $NetBSD: howto.mdwn,v 1.11 2014/12/23 23:25:57 gdt Exp $
     #
     # /usr/pkg/etc/xen/vif-bridge
     #