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remains. Xen is planning to phase out support for PV upstream since it is harder to maintain and has more overhead. Modern x86 CPUs include virtualization extensions that support HVM guests instead of PV guests. In addition, the PV code was i386 only and not as well maintained recently as the HVM code. - Remove the i386-only NATIVE option that was used to disable certain components for PV kernels. These components are now standard as they are on amd64. - Remove !XENHVM bits from PV drivers. - Remove various shims required for XEN (e.g. PT_UPDATES_FLUSH, LOAD_CR3, etc.) - Remove duplicate copy of <xen/features.h>. - Remove unused, i386-only xenstored.h. Differential Revision: https://reviews.freebsd.org/D2362 Reviewed by: royger Tested by: royger (i386/amd64 HVM domU and amd64 PVH dom0) Relnotes: yes
150 lines
5.8 KiB
Groff
150 lines
5.8 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 April 30, 2015
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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 hardware-assisted virtualization (HVM) Xen guest support with
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para-virtualized drivers into an amd64 or i386 kernel,
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place the following lines in 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 hardware-assisted virtualization (HVM) on both i386 and amd64
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kernels.
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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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These para-virtualized 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 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 Mt kmacy@FreeBSD.org
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and
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.An Doug Rabson Aq Mt dfr@FreeBSD.org .
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Further refinements were made by
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.An Justin Gibbs Aq Mt gibbs@FreeBSD.org ,
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.An Adrian Chadd Aq Mt adrian@FreeBSD.org ,
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and
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.An Colin Percival Aq Mt cperciva@FreeBSD.org .
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This manual page was written by
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.An Robert Watson Aq Mt 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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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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