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version 1.147, 2018/07/26 10:47:25 version 1.151, 2018/07/26 11:16:22
Line 6  Introduction Line 6  Introduction
 [![[Xen  [![[Xen
 screenshot]](https://www.netbsd.org/gallery/in-Action/hubertf-xens.png)](https://www.netbsd.org/gallery/in-Action/hubertf-xen.png)  screenshot]](https://www.netbsd.org/gallery/in-Action/hubertf-xens.png)](https://www.netbsd.org/gallery/in-Action/hubertf-xen.png)
   
 Xen is a hypervisor (or virtual machine monitor) for x86 hardware  Xen is a hypervisor for x86 hardware, which supports running multiple guest
 (i686-class or higher), which supports running multiple guest  
 operating systems on a single physical machine.  Xen is a Type 1 or  operating systems on a single physical machine.  Xen is a Type 1 or
 bare-metal hypervisor; one uses the Xen kernel to control the CPU,  bare-metal hypervisor; one uses the Xen kernel to control the CPU,
 memory and console, a dom0 operating system which mediates access to  memory and console, a dom0 operating system which mediates access to
 other hardware (e.g., disks, network, USB), and one or more domU  other hardware (e.g., disks, network, USB), and one or more domU
 operating systems which operate in an unprivileged virtualized  operating systems which operate in an unprivileged virtualized
 environment.  IO requests from the domU systems are forwarded by the  environment.  IO requests from the domU systems are forwarded by the
 hypervisor (Xen) to the dom0 to be fulfilled.  Xen hypervisor to the dom0 to be fulfilled.
   
   Xen supports different styles of guest:
   
   [[!table data="""
   Style of guest  |Supported by NetBSD
   PV              |Yes (dom0, domU)
   HVM             |Yes (domU)
   PVHVM           |No
   PVH             |No
   """]]
   
   In Para-Virtualized (PV) mode, the guest OS does not attempt to access
   hardware directly, but instead makes hypercalls to the hypervisor; PV
   guests must be specifically coded for Xen. In HVM mode, no guest
   modification is required; however, hardware support is required, such
   as VT-x on Intel CPUs and SVM on AMD CPUs.
   
 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.  
   
 Generally any machine that runs NetBSD/amd64 will work with Xen and PV  
 guests.  In theory i386 computers (without x86_64/amd64 support) can  
 be used for Xen <= 4.2, but we have no recent reports of this working  
 (this is a hint).  For HVM guests, hardware support is needed, but it  
 is common on recent machines.  For Intel CPUs, one needs the VT-x  
 extension, shown in "cpuctl identify 0" as VMX.  For AMD CPUs, one  
 needs the AMD-V extensions, shown in "cpuctl identify 0" as SVM.  
 There are further features for IOMMU virtualization, Intel's VT-d and  There are further features for IOMMU virtualization, Intel's VT-d and
 AMD's AMD-Vi.  TODO: Explain whether Xen on NetBSD makes use of these  AMD's AMD-Vi.  TODO: Explain whether Xen on NetBSD makes use of these
 features.  TODO: Review by someone who really understands this.  features.  TODO: Review by someone who really understands this.
   
 Note that a FreeBSD dom0 requires VT-x and VT-d (or equivalent); this  
 is because the FreeBSD dom0 does not run in PV mode.  
   
 At boot, the dom0 kernel is loaded as a module with Xen as the kernel.  At boot, the dom0 kernel is loaded as a module with Xen as the kernel.
 The dom0 can start one or more domUs.  (Booting is explained in detail  The dom0 can start one or more domUs.  (Booting is explained in detail
 in the dom0 section.)  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 domUs under it (NetBSD and other), and also running NetBSD  
 as a domU in a VPS.  
   
 Xen 3.1 in pkgsrc used to support "PCI passthrough", which means that  
 specific PCI devices can be made available to a specific domU instead  
 of the dom0.  This can be useful to let a domU run X11, or access some  
 network interface or other peripheral.  
   
 NetBSD 6 and earlier supported Xen 2; support was removed from NetBSD  
 7.  Xen 2 has been removed from pkgsrc.  
   
 Prerequisites  
 -------------  
   
 Installing NetBSD/Xen is not extremely difficult, but it is more  
 complex than a normal installation of NetBSD.  
 In general, this HOWTO is occasionally overly restrictive about how  
 things must be done, guiding the reader to stay on the established  
 path when there are no known good reasons to stray.  
   
 This HOWTO presumes a basic familiarity with the Xen system  This HOWTO presumes a basic familiarity with the Xen system
 architecture, with installing NetBSD on i386/amd64 hardware, and with  architecture, with installing NetBSD on i386/amd64 hardware, and with
 installing software from pkgsrc.  See also the [Xen  installing software from pkgsrc.  See also the [Xen
 website](http://www.xenproject.org/).  website](http://www.xenproject.org/).
   
   This HOWTO attempts to address both the case of running a NetBSD dom0
   on hardware and running domUs under it (NetBSD and other), and also
   running NetBSD as a domU in a VPS.
   
 Versions of Xen and NetBSD  Versions of Xen and NetBSD
 ==========================  ==========================
   
Line 84  which version to choose.  Versions not i Line 57  which version to choose.  Versions not i
 versions of NetBSD are intentionally ignored.  versions of NetBSD are intentionally ignored.
   
 The term "amd64" is used to refer to both the NetBSD port and to the  The term "amd64" is used to refer to both the NetBSD port and to the
 hardware architecture on which it runs.  (Such hardware is made by  hardware architecture on which it runs.  Such hardware is generally
 both Intel and AMD, and in 2016 a normal PC has this CPU  made by both Intel and AMD, and common on PC computers.
 architecture.)  
   
 Xen versions  Xen versions
 ------------  ------------
Line 157  Architecture Line 129  Architecture
 ------------  ------------
   
 Xen itself can run on i386 (Xen < 4.2) or amd64 hardware (all Xen  Xen itself can run on i386 (Xen < 4.2) or amd64 hardware (all Xen
 versions).  (Practically, almost any computer where one would want to  versions).  Practically, almost any computer where one would want to
 run Xen today supports amd64.)  run Xen today supports amd64.
   
 Xen, the dom0 system, and each domU system can be either i386 or  Xen, the dom0 system, and each domU system can be either i386 or
 amd64.  When building a xenkernel package, one obtains an i386 Xen  amd64.  When building a xenkernel package, one obtains an i386 Xen
Line 168  be i386.  With an amd64 Xen kernel, an a Line 140  be i386.  With an amd64 Xen kernel, an a
 work, and an i386 dom0 kernel should in theory work.  An amd64  work, and an i386 dom0 kernel should in theory work.  An amd64
 Xen/dom0 is known to support both i386 and amd64 domUs.  Xen/dom0 is known to support both i386 and amd64 domUs.
   
 i386 dom0 and domU kernels must be PAE (except for an i386 Xen 3.1  i386 dom0 and domU kernels must be PAE. PAE kernels are included in
 kernel, where one can use non-PAE for dom0 and all domUs); PAE kernels  the NetBSD default build.
 are included in the NetBSD default build.  (Note that emacs (at least)  
 fails if run on i386 with PAE when built without, and vice versa,  
 presumably due to bugs in the undump code.)  
   
 Because of the above, the standard approach is to use an amd64 Xen  Because of the above, the standard approach is to use an amd64 Xen
 kernel and NetBSD/amd64 for the dom0.  For domUs, NetBSD/i386 (with  kernel and NetBSD/amd64 for the dom0.  For domUs, NetBSD/i386 (with
Line 194  caution that the total situation is comp Line 163  caution that the total situation is comp
 understood. On top of that caution, the post is about Linux, not  understood. On top of that caution, the post is about Linux, not
 NetBSD.  TODO: Include link to benchmarks, if someone posts them.  NetBSD.  TODO: Include link to benchmarks, if someone posts them.
   
 Stability  
 ---------  
   
 Mostly, NetBSD as a dom0 or domU is quite stable. However, just like every  
 other architecture, there are some open PRs indicating problems.  
   
 Note also that there are issues with sparse vnd(4) instances, but  
 these are not about Xen -- they just are noticed with sparse vnd(4)  
 instances in support of virtual disks in a dom0.  
   
 Recommendation  
 --------------  
   
 Therefore, this HOWTO recommends running xenkernel46, xl, the NetBSD 7  
 stable branch, and therefore to use an amd64 kernel as the dom0.  
 Either the i386PAE or amd64 version of NetBSD may be used as domUs.  
   
 A tentative replacement recommendation is xenkernel48, xl, and NetBSD  
 8.  
   
 Because bugs are fixed quite often, and because of Xen security  
 advisories, it is good to stay up to date with NetBSD (tracking a  
 stable branch), with the Xen kernel (tracking a Xen version via  
 pkgsrc), and with the Xen tools.  Specifically, NetBSD (-7 and  
 -current) got an important fix affecting dom0/domU timesharing in  
 November, 2015, and xentools46 got a fix to enable Ubuntu guests to  
 boot in December, 2016.  
   
 NetBSD as a dom0  NetBSD as a dom0
 ================  ================
   
Removed from v.1.147  
changed lines
  Added in v.1.151

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