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186 lines
6.8 KiB
Groff
186 lines
6.8 KiB
Groff
.\" Copyright (c) 2010 Robert N. M. Watson
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.\" All rights reserved.
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.\"
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.\" This software was developed by SRI International and the University of
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.\" Cambridge Computer Laboratory under DARPA/AFRL contract FA8750-10-C-0237
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.\" ("CTSRD"), as part of the DARPA CRASH research program.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 17, 2010
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.Dt XEN 4
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.Os
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.Sh NAME
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.Nm xen
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.Nd Xen Hypervisor Guest (DomU) Support
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.Sh SYNOPSIS
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To compile para-virtualized (PV) Xen guest support into an i386 kernel, place
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the following lines in your kernel configuration file:
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.Bd -ragged -offset indent
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.Cd "options PAE"
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.Cd "options XEN"
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.Cd "nooptions NATIVE"
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.Ed
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.Pp
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To compile hardware-assisted virtualization (HVM) Xen guest support with
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para-virtualized drivers into an amd64 kernel, place the following lines in
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your kernel configuration file:
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.Bd -ragged -offset indent
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.Cd "options XENHVM"
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.Cd "device xenpci"
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.Ed
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.Sh DESCRIPTION
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The Xen Hypervisor allows multiple virtual machines to be run on a single
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computer system.
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When first released, Xen required that i386 kernels be compiled
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"para-virtualized" as the x86 instruction set was not fully virtualizable.
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Primarily, para-virtualization modifies the virtual memory system to use
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hypervisor calls (hypercalls) rather than direct hardware instructions to
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modify the TLB, although para-virtualized device drivers were also required
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to access resources such as virtual network interfaces and disk devices.
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.Pp
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With later instruction set extensions from AMD and Intel to support fully
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virtualizable instructions, unmodified virtual memory systems can also be
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supported; this is referred to as hardware-assisted virtualization (HVM).
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HVM configurations may either rely on transparently emulated hardware
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peripherals, or para-virtualized drivers, which are aware of virtualization,
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and hence able to optimize certain behaviors to improve performance or
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semantics.
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.Pp
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.Fx
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supports a fully para-virtualized (PV) kernel on the i386 architecture using
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.Cd "options XEN"
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and
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.Cd "nooptions NATIVE" ;
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currently, this requires use of a PAE kernel, enabled via
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.Cd "options PAE" .
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.Pp
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.Fx
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supports hardware-assisted virtualization (HVM) on both the i386 and amd64
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kernels; however, PV device drivers with an HVM kernel are only supported on
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the amd64 architecture, and require
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.Cd "options XENHVM"
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and
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.Cd "device xenpci" .
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.Pp
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Para-virtualized device drivers are required in order to support certain
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functionality, such as processing management requests, returning idle
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physical memory pages to the hypervisor, etc.
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.Ss Xen DomU device drivers
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Xen para-virtualized drivers are automatically added to the kernel if a PV
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kernel is compiled using
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.Cd "options XEN" ;
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for HVM environments,
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.Cd "options XENHVM"
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and
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.Cd "device xenpci"
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are required.
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The follow drivers are supported:
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.Bl -hang -offset indent -width blkfront
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.It Nm balloon
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Allow physical memory pages to be returned to the hypervisor as a result of
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manual tuning or automatic policy.
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.It Nm blkback
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Exports local block devices or files to other Xen domains where they can
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then be imported via
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.Nm blkfront .
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.It Nm blkfront
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Import block devices from other Xen domains as local block devices, to be
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used for file systems, swap, etc.
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.It Nm console
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Export the low-level system console via the Xen console service.
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.It Nm control
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Process management operations from Domain 0, including power off, reboot,
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suspend, crash, and halt requests.
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.It Nm evtchn
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Expose Xen events via the
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.Pa /dev/xen/evtchn
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special device.
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.It Nm netback
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Export local network interfaces to other Xen domains where they can be
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imported via
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.Nm netfront .
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.It Nm netfront
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Import network interfaces from other Xen domains as local network interfaces,
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which may be used for IPv4, IPv6, etc.
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.It Nm pcifront
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Allow physical PCI devices to be passed through into a PV domain.
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.It Nm xenpci
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Represents the Xen PCI device, an emulated PCI device that is exposed to
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HVM domains.
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This device allows detection of the Xen hypervisor, and provides interrupt
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and shared memory services required to interact with the hypervisor.
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.El
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.Ss Performance considerations
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In general, PV drivers will perform better than emulated hardware, and are
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the recommended configuration for HVM installations.
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.Pp
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Using a hypervisor introduces a second layer of scheduling that may limit the
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effectiveness of certain
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.Fx
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scheduling optimisations.
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Among these is adaptive locking, which is no longer able to determine whether
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a thread holding a lock is in execution.
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It is recommended that adaptive locking be disabled when using Xen:
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.Bd -unfilled -offset indent
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.Cd "options NO_ADAPTIVE_MUTEXES"
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.Cd "options NO_ADAPTIVE_RWLOCKS"
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.Cd "options NO_ADAPTIVE_SX"
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.Ed
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.Sh SEE ALSO
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.Xr pae 4
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.Sh HISTORY
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Support for
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.Nm
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first appeared in
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.Fx 8.1 .
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.Sh AUTHORS
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.An -nosplit
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.Fx
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support for Xen was first added by
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.An Kip Macy Aq kmacy@FreeBSD.org
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and
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.An Doug Rabson Aq dfr@FreeBSD.org .
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Further refinements were made by
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.An Justin Gibbs Aq gibbs@FreeBSD.org ,
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.An Adrian Chadd Aq adrian@FreeBSD.org ,
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and
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.An Colin Percival Aq cperciva@FreeBSD.org .
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This manual page was written by
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.An Robert Watson Aq rwatson@FreeBSD.org .
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.Sh BUGS
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.Fx
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is only able to run as a Xen guest (DomU) and not as a Xen host (Dom0).
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.Pp
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A fully para-virtualized (PV) kernel is only supported on i386, and not
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amd64.
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.Pp
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Para-virtualized drivers under hardware-assisted virtualization (HVM) kernel
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are only supported on amd64, not i386.
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.Pp
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As of this release, Xen PV DomU support is not heavily tested; instability
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has been reported during VM migration of PV kernels.
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.Pp
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Certain PV driver features, such as the balloon driver, are under-exercised.
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