openstack, kvm, qemu-kvm以及libvirt之间的关系

不念不忘少年蓝@ 2022-07-28 04:30 159阅读 0赞

# 虚拟化类型 #

## 全虚拟化（Full Virtualization) ##

全虚拟化也成为原始虚拟化技术，该模型使用虚拟机协调guest操作系统和原始硬件，VMM在guest操作系统和裸硬件之间用于工作协调，一些受保护指令必须由Hypervisor（虚拟机管理程序）来捕获处理。

[![22242080_1.gif][]][22242080_1.gif]

图1 全虚拟化模型

全虚拟化的运行速度要快于硬件模拟，但是性能方面不如裸机，因为Hypervisor需要占用一些资源。

## 半虚拟化（Para Virtualization） ##

半虚拟化是另一种类似于全虚拟化的技术，它使用Hypervisor分享存取底层的硬件，但是它的guest操作系统集成了虚拟化方面的代码。该方法无需重新编译或引起陷阱，因为操作系统自身能够与虚拟进程进行很好的协作。

[![22242080_2.gif][]][22242080_2.gif]

图2 半虚拟化模型

半虚拟化需要guest操作系统做一些修改，使guest操作系统意识到自己是处于虚拟化环境的，但是半虚拟化提供了与原操作系统相近的性能。

# 虚拟化技术 #

## KVM(Kernel-based Virtual Machine)基于内核的虚拟机 ##

KVM是集成到Linux内核的Hypervisor，是X86架构且硬件支持虚拟化技术（Intel VT或AMD-V）的Linux的全虚拟化解决方案。它是Linux的一个很小的模块，利用Linux做大量的事，如任务调度、内存管理与硬件设备交互等。

[![kvm_E8_99_9A_E6_8B_9F_E5_8C_96_E5_B9_B3_E5_8F_B0_E6_9E_B6_E6_9E_84.jpg][]][kvm_E8_99_9A_E6_8B_9F_E5_8C_96_E5_B9_B3_E5_8F_B0_E6_9E_B6_E6_9E_84.jpg]

图3 KVM虚拟化平台架构

KVM是linux内核的模块，它需要CPU的支持，采用硬件辅助虚拟化技术Intel-VT，AMD-V，内存的相关如Intel的EPT和AMD的RVI技术，Guest OS的CPU指令不用再经过Qemu转译，直接运行，大大提高了速度，KVM通过/dev/kvm暴露接口，用户态程序可以通过ioctl函数来访问这个接口。见如下伪代码：

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      <code style="padding:0px!important; margin:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="padding:0px!important; margin:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; font-size:1em!important; min-height:auto!important; color:rgb(0,102,153)!important">case</code>&nbsp; 
      <code style="padding:0px!important; margin:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important">KVM_EXIT_HLT:</code> 
     </div> 
     <div style="white-space:nowrap; padding:0px 1em!important; margin:0px!important; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-size:1em!important; min-height:auto!important"> 
      <code style="padding:0px!important; margin:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="padding:0px!important; margin:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important">}</code> 
     </div> 
     <div style="white-space:nowrap; padding:0px 1em!important; margin:0px!important; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-size:1em!important; min-height:auto!important"> 
      <code style="padding:0px!important; margin:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important">}</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

KVM内核模块本身只能提供CPU和内存的虚拟化，所以它必须结合QEMU才能构成一个完成的虚拟化技术，这就是下面要说的qemu-kvm。

# **qemu-kvm** #

Qemu将KVM整合进来，通过ioctl调用/dev/kvm接口，将有关CPU指令的部分交由内核模块来做。kvm负责cpu虚拟化\+内存虚拟化，实现了cpu和内存的虚拟化，但kvm不能模拟其他设备。qemu模拟IO设备（网卡，磁盘等），kvm加上qemu之后就能实现真正意义上服务器虚拟化。因为用到了上面两个东西，所以称之为qemu-kvm。

Qemu模拟其他的硬件，如Network, Disk，同样会影响这些设备的性能，于是又产生了pass through半虚拟化设备virtio\_blk, virtio\_net，提高设备性能。

[![wKiom1WdDc2CEwy6AAGPf4VzQao172.jpg][]][wKiom1WdDc2CEwy6AAGPf4VzQao172.jpg]

## Xen ##

Xen是第一类运行在裸机上的虚拟化管理程序(Hypervisor)。它支持全虚拟化和半虚拟化,Xen支持hypervisor和虚拟机互相通讯，而且提供在所有Linux版本上的免费产品，包括Red Hat Enterprise Linux和SUSE Linux Enterprise Server。Xen最重要的优势在于半虚拟化，此外未经修改的操作系统也可以直接在xen上运行(如Windows)，能让虚拟机有效运行而不需要仿真，因此虚拟机能感知到hypervisor，而不需要模拟虚拟硬件，从而能实现高性能。

[![Xen_E8_99_9A_E6_8B_9F_E5_8C_96_E5_B9_B3_E5_8F_B0_E6_9E_B6_E6_9E_84.jpg][]][Xen_E8_99_9A_E6_8B_9F_E5_8C_96_E5_B9_B3_E5_8F_B0_E6_9E_B6_E6_9E_84.jpg]

图4 Xen虚拟化平台架构

## QEMU ##

QEMU是一套由Fabrice Bellard所编写的模拟处理器的自由软件。它与Bochs，PearPC近似，但其具有某些后两者所不具备的特性，如高速度及跨平台的特性。经由kqemu这个开源的加速器，QEMU能模拟至接近真实电脑的速度。

Qemu是一个模拟器，它向Guest OS模拟CPU和其他硬件，Guest OS认为自己和硬件直接打交道，其实是同Qemu模拟出来的硬件打交道，Qemu将这些指令转译给真正的硬件。

由于所有的指令都要从Qemu里面过一手，因而性能较差[。![wKiom1WdDYyjiVZiAAECBtAEQ5E590.jpg][]][wKiom1WdDYyjiVZiAAECBtAEQ5E590.jpg]

libvirt

libvirt是目前使用最为广泛的对KVM虚拟机进行管理的工具和API。Libvirtd是一个daemon进程，可以被本地的virsh调用，也可以被远程的virsh调用，Libvirtd调用qemu-kvm操作虚拟机。

[![wKioL1WdD72RRy8mAAIuDm6sVAY591.jpg][]][wKioL1WdD72RRy8mAAIuDm6sVAY591.jpg]

参考：http://blog.csdn.net/gaoxingnengjisuan/article/details/9674315

**Libvirt****体系结构**

没有使用libvirt的虚拟机管理方式如下图所示：

![Center][]

为支持各种虚拟机监控程序的可扩展性，libvirt实施一种基于驱动程序的架构，该架构允许一种通用的API以通用方式为大量潜在的虚拟机监控程序提供服务。下图展示了libvirt API与相关驱动程序的层次结构。这里也需要注意，libvirtd提供从远程应用程序访问本地域的方式。

![Center 1][]

libvirt的控制方式有两种：

1）管理应用程序和域位于同一节点上。管理应用程序通过libvirt工作，以控制本地域。

![Center 2][]

2）管理应用程序和域位于不同节点上。该模式使用一种运行于远程节点上、名为libvirtd的特殊守护进程。当在新节点上安装libvirt时该程序会自动启动，且可自动确定本地虚拟机监控程序并为其安装驱动程序。该管理应用程序通过一种通用协议从本地libvirt连接到远程libvirtd。

![Center 3][]

## KVM和QEMU的关系 ##

准确来说，KVM是Linux kernel的一个模块。可以用命令modprobe去加载KVM模块。加载了模块后，才能进一步通过其他工具创建虚拟机。但仅有KVM模块是 远远不够的，因为用户无法直接控制内核模块去作事情,你还必须有一个运行在用户空间的工具才行。这个用户空间的工具，kvm开发者选择了已经成型的开源虚拟化软件 QEMU。说起来QEMU也是一个虚拟化软件。它的特点是可虚拟不同的CPU。比如说在x86的CPU上可虚拟一个Power的CPU，并可利用它编译出可运行在Power上的程序。KVM使用了QEMU的一部分，并稍加改造，就成了可控制KVM的用户空间工具了。所以你会看到，官方提供的KVM下载有两大部分(qemu和kvm)三个文件(KVM模块、QEMU工具以及二者的合集)。也就是说，你可以只升级KVM模块，也可以只升级QEMU工具。这就是KVM和QEMU 的关系。

[![KVM_E5_92_8CQEMU_E5_85_B3_E7_B3_BB.png][]][KVM_E5_92_8CQEMU_E5_85_B3_E7_B3_BB.png]

图5 KVM和QEMU关系

# openstack, kvm, qemu-kvm以及libvirt之间的关系 #

KVM是最底层的hypervisor，它是用来模拟CPU的运行，它缺少了对network和周边I/O的支持，所以我们是没法直接用它的。

QEMU-KVM就是一个完整的模拟器，它是构建基于KVM上面的，它提供了完整的网络和I/O支持。

Openstack不会直接控制qemu-kvm，它会用一个叫libvirt的库去间接控制qemu-kvm。libvirt提供了跨VM平台的功能，它可以控制除了QEMU之外的模拟器，包括vmware, virtualbox， xen等等。

所以为了openstack的跨VM性，所以openstack只会用libvirt而不直接用qemu-kvm。libvirt还提供了一些高级的功能，例如pool/vol管理。

[![openstack_libvirt_qemu_kvm.png][]][openstack_libvirt_qemu_kvm.png]

原文：https://huangwei.me/wiki/tech\_cloud\_kvm\_qemu\_libvirt\_openstack.html

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[Center 2]: /images/20220727/28f2be16174f47179dae65ee1dcc5e06.png
[Center 3]: /images/20220727/fd1b914268864c7f9f6029eaea25e5a6.png
[KVM_E5_92_8CQEMU_E5_85_B3_E7_B3_BB.png]: https://huangwei.me/wiki/image/KVM%E5%92%8CQEMU%E5%85%B3%E7%B3%BB.png
[openstack_libvirt_qemu_kvm.png]: https://huangwei.me/wiki/image/openstack_libvirt_qemu_kvm.png