QEMU源码全解析 —— 内存虚拟化（2）

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本文介绍了QEMU内存管理模型中的关键结构AddressSpace和MemoryRegion。AddressSpace表示虚拟机或CPU的地址空间，包括全局、CPU和设备视角。MemoryRegion构成无环图模拟内存，用于描述内存区域。QEMU通过MemoryRegion和AddressSpace实现内存的正向管理和反向查找。此外，MemoryRegion与AddressSpace的关系以及它们在QEMU内存初始化中的作用被详细阐述。
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接前一篇文章： QEMU源码全解析 —— 内存虚拟化（1）

本文内容参考：

《趣谈 Linux操作系统 》 —— 刘超， 极客时间

《 QEMU /KVM》源码解析与应用 —— 李强，机械工业出版社

QEMU内存管理模型

特此致谢！

QEMU 内存初始化

1. 基本结构

在开始介绍内存初始化的时候，首先需要对QEMU中几个与内存相关的数据结构进行介绍。

（1）AddressSpace

首先是AddressSpace结构。 AddressSpace结构用来表示一个虚拟机或者虚拟CPU能够访问的所有物理地址 。注意，这里的“访问”和“能够访问”并不是一回事。与进程的地址空间一样，一个进程的虚拟地址空间为4GB（32位），但这并不是说操作系统需要为进程分配这么大的空间。同样，QEMU中的AddressSpace表示的是一段地址空间，整个系统可以有一个全局的地址空间，CPU可以有自己的地址空间视角，设备也可以有自己的地址空间视角。

AddressSpace的意义从名字就可以看出，是针对整个地址空间，最顶层的内存管理结构。对于x86架构来说，其实是有两种寻址方式的，一种是memory、一种是IO。相应的QEMU会维护两个AddressSpace结构，address_space_memory和address_space_io。

AddressSpace的定义在include/qemu/typedefs.h中，如下： 

typedef struct AddressSpace AddressSpace;

而struct AddressSpace的定义在include/exec/memory.h中，代码如下： 

  1. /**
  2.  * struct AddressSpace: describes a mapping of addresses to #MemoryRegion objects
  3.  */
  4. struct AddressSpace {
  5.     /* private: */
  6.     struct rcu_head rcu;
  7.     char *name;
  8.     MemoryRegion *root;
  9.  
  10.     /* Accessed via RCU.  */
  11.     struct FlatView *current_map;
  12.  
  13.     int ioeventfd_nb;
  14.     struct MemoryRegionIoeventfd *ioeventfds;
  15.     QTAILQ_HEAD(, MemoryListener) listeners;
  16.     QTAILQ_ENTRY(AddressSpace) address_spaces_link;
  17. };

根据struct AddressSpace的注释，此结构描述了一个到MemoryRegion对象的地址映射。

其中：

  • MemoryRegion *root成员表示AddressSpace对应的一个根MemoryRegion。
  • struct FlatView *current_map成员表示该地址空间是一个平坦模式下的视图。
  • QEMU的其它子系统可以注册地址变更的事件，所有注册的信息都通过QTAILQ_HEAD(, MemoryListener) listeners成员连接起来。
  • 所有的AddressSpace通过address_spaces_link这个node连接起来（QTAILQ_ENTRY(AddressSpace) address_spaces_link），链表头是address_spaces。

AddressSpace的定义其实比较简单，有两个QEMU内存管理框架的核心结构，就是MemoryRegion和FlatView，两者配合使用，MemoryRegion是基础（MemoryRegion），用于正向管理；Flatview用于反向查找。

对于一个地址空间（AddressSpace），会有多个内存区域（MemoryRegion）组成树型结构。这其中，root是这棵树的根。另外，还有一个MemoryListener链表，当内存区域发生变化的时候，需要做一些动作，使得用户态和内核态能够协同，就是由这些MemoryListener来完成的。

在QEMU monitor中输入“info qtree”，可以看到所有的AddressSpace，如下所示： 

  1. $ qemu-system-x86_64 -vnc :1 -monitor stdio
  2. QEMU 6.2.0 monitor - type 'help' for more information
  3. (qemu) info mtree
  4. address-space: memory
  5.   0000000000000000-ffffffffffffffff (prio 0, i/o): system
  6.     0000000000000000-0000000007ffffff (prio 0, ram): alias ram-below-4g @pc.ram 0000000000000000-0000000007ffffff
  7.     0000000000000000-ffffffffffffffff (prio -1, i/o): pci
  8.       00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
  9.       00000000000c0000-00000000000dffff (prio 1, rom): pc.rom
  10.       00000000000e0000-00000000000fffff (prio 1, rom): alias isa-bios @pc.bios 0000000000020000-000000000003ffff
  11.       00000000fd000000-00000000fdffffff (prio 1, ram): vga.vram
  12.       00000000feb80000-00000000feb9ffff (prio 1, i/o): e1000-mmio
  13.       00000000febb0000-00000000febb0fff (prio 1, i/o): vga.mmio
  14.         00000000febb0000-00000000febb017f (prio 0, i/o): edid
  15.         00000000febb0400-00000000febb041f (prio 0, i/o): vga ioports remapped
  16.         00000000febb0500-00000000febb0515 (prio 0, i/o): bochs dispi interface
  17.         00000000febb0600-00000000febb0607 (prio 0, i/o): qemu extended regs
  18.       00000000fffc0000-00000000ffffffff (prio 0, rom): pc.bios
  19.     00000000000a0000-00000000000bffff (prio 1, i/o): alias smram-region @pci 00000000000a0000-00000000000bffff
  20.     00000000000c0000-00000000000c3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000c0000-00000000000c3fff
  21.     00000000000c4000-00000000000c7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000c4000-00000000000c7fff
  22.     00000000000c8000-00000000000cbfff (prio 1, ram): alias pam-rom @pc.ram 00000000000c8000-00000000000cbfff
  23.     00000000000cb000-00000000000cdfff (prio 1000, ram): alias kvmvapic-rom @pc.ram 00000000000cb000-00000000000cdfff
  24.     00000000000cc000-00000000000cffff (prio 1, ram): alias pam-rom @pc.ram 00000000000cc000-00000000000cffff
  25.     00000000000d0000-00000000000d3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000d0000-00000000000d3fff
  26.     00000000000d4000-00000000000d7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000d4000-00000000000d7fff
  27.     00000000000d8000-00000000000dbfff (prio 1, ram): alias pam-rom @pc.ram 00000000000d8000-00000000000dbfff
  28.     00000000000dc000-00000000000dffff (prio 1, ram): alias pam-rom @pc.ram 00000000000dc000-00000000000dffff
  29.     00000000000e0000-00000000000e3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000e0000-00000000000e3fff
  30.     00000000000e4000-00000000000e7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000e4000-00000000000e7fff
  31.     00000000000e8000-00000000000ebfff (prio 1, ram): alias pam-ram @pc.ram 00000000000e8000-00000000000ebfff
  32.     00000000000ec000-00000000000effff (prio 1, ram): alias pam-ram @pc.ram 00000000000ec000-00000000000effff
  33.     00000000000f0000-00000000000fffff (prio 1, ram): alias pam-rom @pc.ram 00000000000f0000-00000000000fffff
  34.     00000000fec00000-00000000fec00fff (prio 0, i/o): ioapic
  35.     00000000fed00000-00000000fed003ff (prio 0, i/o): hpet
  36.     00000000fee00000-00000000feefffff (prio 4096, i/o): apic-msi
  37.  
  38. address-space: I/O
  39.   0000000000000000-000000000000ffff (prio 0, i/o): io
  40.     0000000000000000-0000000000000007 (prio 0, i/o): dma-chan
  41.     0000000000000008-000000000000000f (prio 0, i/o): dma-cont
  42.     0000000000000020-0000000000000021 (prio 0, i/o): pic
  43.     0000000000000040-0000000000000043 (prio 0, i/o): pit
  44.     0000000000000060-0000000000000060 (prio 0, i/o): i8042-data
  45.     0000000000000061-0000000000000061 (prio 0, i/o): pcspk
  46.     0000000000000064-0000000000000064 (prio 0, i/o): i8042-cmd
  47.     0000000000000070-0000000000000071 (prio 0, i/o): rtc
  48.       0000000000000070-0000000000000070 (prio 0, i/o): rtc-index
  49.     000000000000007e-000000000000007f (prio 0, i/o): kvmvapic
  50.     0000000000000080-0000000000000080 (prio 0, i/o): ioport80
  51.     0000000000000081-0000000000000083 (prio 0, i/o): dma-page
  52.     0000000000000087-0000000000000087 (prio 0, i/o): dma-page
  53.     0000000000000089-000000000000008b (prio 0, i/o): dma-page
  54.     000000000000008f-000000000000008f (prio 0, i/o): dma-page
  55.     0000000000000092-0000000000000092 (prio 0, i/o): port92
  56.     00000000000000a0-00000000000000a1 (prio 0, i/o): pic
  57.     00000000000000b2-00000000000000b3 (prio 0, i/o): apm-io
  58.     00000000000000c0-00000000000000cf (prio 0, i/o): dma-chan
  59.     00000000000000d0-00000000000000df (prio 0, i/o): dma-cont
  60.     00000000000000f0-00000000000000f0 (prio 0, i/o): ioportF0
  61.     0000000000000170-0000000000000177 (prio 0, i/o): ide
  62.     00000000000001ce-00000000000001d1 (prio 0, i/o): vbe
  63.     00000000000001f0-00000000000001f7 (prio 0, i/o): ide
  64.     0000000000000376-0000000000000376 (prio 0, i/o): ide
  65.     0000000000000378-000000000000037f (prio 0, i/o): parallel
  66.     00000000000003b4-00000000000003b5 (prio 0, i/o): vga
  67.     00000000000003ba-00000000000003ba (prio 0, i/o): vga
  68.     00000000000003c0-00000000000003cf (prio 0, i/o): vga
  69.     00000000000003d4-00000000000003d5 (prio 0, i/o): vga
  70.     00000000000003da-00000000000003da (prio 0, i/o): vga
  71.     00000000000003f1-00000000000003f5 (prio 0, i/o): fdc
  72.     00000000000003f6-00000000000003f6 (prio 0, i/o): ide
  73.     00000000000003f7-00000000000003f7 (prio 0, i/o): fdc
  74.     00000000000003f8-00000000000003ff (prio 0, i/o): serial
  75.     00000000000004d0-00000000000004d0 (prio 0, i/o): elcr
  76.     00000000000004d1-00000000000004d1 (prio 0, i/o): elcr
  77.     0000000000000510-0000000000000511 (prio 0, i/o): fwcfg
  78.     0000000000000514-000000000000051b (prio 0, i/o): fwcfg.dma
  79.     0000000000000600-000000000000063f (prio 0, i/o): piix4-pm
  80.       0000000000000600-0000000000000603 (prio 0, i/o): acpi-evt
  81.       0000000000000604-0000000000000605 (prio 0, i/o): acpi-cnt
  82.       0000000000000608-000000000000060b (prio 0, i/o): acpi-tmr
  83.     0000000000000700-000000000000073f (prio 0, i/o): pm-smbus
  84.     0000000000000cf8-0000000000000cfb (prio 0, i/o): pci-conf-idx
  85.     0000000000000cf9-0000000000000cf9 (prio 1, i/o): piix3-reset-control
  86.     0000000000000cfc-0000000000000cff (prio 0, i/o): pci-conf-data
  87.     0000000000005658-0000000000005658 (prio 0, i/o): vmport
  88.     000000000000ae00-000000000000ae17 (prio 0, i/o): acpi-pci-hotplug
  89.     000000000000af00-000000000000af1f (prio 0, i/o): acpi-cpu-hotplug
  90.     000000000000afe0-000000000000afe3 (prio 0, i/o): acpi-gpe0
  91.     000000000000c000-000000000000c03f (prio 1, i/o): e1000-io
  92.     000000000000c040-000000000000c04f (prio 1, i/o): piix-bmdma-container
  93.       000000000000c040-000000000000c043 (prio 0, i/o): piix-bmdma
  94.       000000000000c044-000000000000c047 (prio 0, i/o): bmdma
  95.       000000000000c048-000000000000c04b (prio 0, i/o): piix-bmdma
  96.       000000000000c04c-000000000000c04f (prio 0, i/o): bmdma
  97.  
  98. address-space: cpu-memory-0
  99.   0000000000000000-ffffffffffffffff (prio 0, i/o): system
  100.     0000000000000000-0000000007ffffff (prio 0, ram): alias ram-below-4g @pc.ram 0000000000000000-0000000007ffffff
  101.     0000000000000000-ffffffffffffffff (prio -1, i/o): pci
  102.       00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
  103.       00000000000c0000-00000000000dffff (prio 1, rom): pc.rom
  104.       00000000000e0000-00000000000fffff (prio 1, rom): alias isa-bios @pc.bios 0000000000020000-000000000003ffff
  105.       00000000fd000000-00000000fdffffff (prio 1, ram): vga.vram
  106.       00000000feb80000-00000000feb9ffff (prio 1, i/o): e1000-mmio
  107.       00000000febb0000-00000000febb0fff (prio 1, i/o): vga.mmio
  108.         00000000febb0000-00000000febb017f (prio 0, i/o): edid
  109.         00000000febb0400-00000000febb041f (prio 0, i/o): vga ioports remapped
  110.         00000000febb0500-00000000febb0515 (prio 0, i/o): bochs dispi interface
  111.         00000000febb0600-00000000febb0607 (prio 0, i/o): qemu extended regs
  112.       00000000fffc0000-00000000ffffffff (prio 0, rom): pc.bios
  113.     00000000000a0000-00000000000bffff (prio 1, i/o): alias smram-region @pci 00000000000a0000-00000000000bffff
  114.     00000000000c0000-00000000000c3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000c0000-00000000000c3fff
  115.     00000000000c4000-00000000000c7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000c4000-00000000000c7fff
  116.     00000000000c8000-00000000000cbfff (prio 1, ram): alias pam-rom @pc.ram 00000000000c8000-00000000000cbfff
  117.     00000000000cb000-00000000000cdfff (prio 1000, ram): alias kvmvapic-rom @pc.ram 00000000000cb000-00000000000cdfff
  118.     00000000000cc000-00000000000cffff (prio 1, ram): alias pam-rom @pc.ram 00000000000cc000-00000000000cffff
  119.     00000000000d0000-00000000000d3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000d0000-00000000000d3fff
  120.     00000000000d4000-00000000000d7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000d4000-00000000000d7fff
  121.     00000000000d8000-00000000000dbfff (prio 1, ram): alias pam-rom @pc.ram 00000000000d8000-00000000000dbfff
  122.     00000000000dc000-00000000000dffff (prio 1, ram): alias pam-rom @pc.ram 00000000000dc000-00000000000dffff
  123.     00000000000e0000-00000000000e3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000e0000-00000000000e3fff
  124.     00000000000e4000-00000000000e7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000e4000-00000000000e7fff
  125.     00000000000e8000-00000000000ebfff (prio 1, ram): alias pam-ram @pc.ram 00000000000e8000-00000000000ebfff
  126.     00000000000ec000-00000000000effff (prio 1, ram): alias pam-ram @pc.ram 00000000000ec000-00000000000effff
  127.     00000000000f0000-00000000000fffff (prio 1, ram): alias pam-rom @pc.ram 00000000000f0000-00000000000fffff
  128.     00000000fec00000-00000000fec00fff (prio 0, i/o): ioapic
  129.     00000000fed00000-00000000fed003ff (prio 0, i/o): hpet
  130.     00000000fee00000-00000000feefffff (prio 4096, i/o): apic-msi
  131.  
  132. address-space: cpu-smm-0
  133.   0000000000000000-ffffffffffffffff (prio 0, i/o): memory
  134.     0000000000000000-00000000ffffffff (prio 1, i/o): alias smram @smram 0000000000000000-00000000ffffffff
  135.     0000000000000000-ffffffffffffffff (prio 0, i/o): alias memory @system 0000000000000000-ffffffffffffffff
  136.  
  137. address-space: i440FX
  138.   0000000000000000-ffffffffffffffff (prio 0, i/o): bus master container
  139.  
  140. address-space: PIIX3
  141.   0000000000000000-ffffffffffffffff (prio 0, i/o): bus master container
  142.  
  143. address-space: VGA
  144.   0000000000000000-ffffffffffffffff (prio 0, i/o): bus master container
  145.  
  146. address-space: e1000
  147.   0000000000000000-ffffffffffffffff (prio 0, i/o): bus master container
  148.     0000000000000000-ffffffffffffffff (prio 0, i/o): alias bus master @system 0000000000000000-ffffffffffffffff
  149.  
  150. address-space: piix3-ide
  151.   0000000000000000-ffffffffffffffff (prio 0, i/o): bus master container
  152.  
  153. address-space: PIIX4_PM
  154.   0000000000000000-ffffffffffffffff (prio 0, i/o): bus master container
  155.  
  156. memory-region: pc.ram
  157.   0000000000000000-0000000007ffffff (prio 0, ram): pc.ram
  158.  
  159. memory-region: pc.bios
  160.   00000000fffc0000-00000000ffffffff (prio 0, rom): pc.bios
  161.  
  162. memory-region: pci
  163.   0000000000000000-ffffffffffffffff (prio -1, i/o): pci
  164.     00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
  165.     00000000000c0000-00000000000dffff (prio 1, rom): pc.rom
  166.     00000000000e0000-00000000000fffff (prio 1, rom): alias isa-bios @pc.bios 0000000000020000-000000000003ffff
  167.     00000000fd000000-00000000fdffffff (prio 1, ram): vga.vram
  168.     00000000feb80000-00000000feb9ffff (prio 1, i/o): e1000-mmio
  169.     00000000febb0000-00000000febb0fff (prio 1, i/o): vga.mmio
  170.       00000000febb0000-00000000febb017f (prio 0, i/o): edid
  171.       00000000febb0400-00000000febb041f (prio 0, i/o): vga ioports remapped
  172.       00000000febb0500-00000000febb0515 (prio 0, i/o): bochs dispi interface
  173.       00000000febb0600-00000000febb0607 (prio 0, i/o): qemu extended regs
  174.     00000000fffc0000-00000000ffffffff (prio 0, rom): pc.bios
  175.  
  176. memory-region: smram
  177.   0000000000000000-00000000ffffffff (prio 0, i/o): smram
  178.     00000000000a0000-00000000000bffff (prio 0, ram): alias smram-low @pc.ram 00000000000a0000-00000000000bffff
  179.  
  180. memory-region: system
  181.   0000000000000000-ffffffffffffffff (prio 0, i/o): system
  182.     0000000000000000-0000000007ffffff (prio 0, ram): alias ram-below-4g @pc.ram 0000000000000000-0000000007ffffff
  183.     0000000000000000-ffffffffffffffff (prio -1, i/o): pci
  184.       00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
  185.       00000000000c0000-00000000000dffff (prio 1, rom): pc.rom
  186.       00000000000e0000-00000000000fffff (prio 1, rom): alias isa-bios @pc.bios 0000000000020000-000000000003ffff
  187.       00000000fd000000-00000000fdffffff (prio 1, ram): vga.vram
  188.       00000000feb80000-00000000feb9ffff (prio 1, i/o): e1000-mmio
  189.       00000000febb0000-00000000febb0fff (prio 1, i/o): vga.mmio
  190.         00000000febb0000-00000000febb017f (prio 0, i/o): edid
  191.         00000000febb0400-00000000febb041f (prio 0, i/o): vga ioports remapped
  192.         00000000febb0500-00000000febb0515 (prio 0, i/o): bochs dispi interface
  193.         00000000febb0600-00000000febb0607 (prio 0, i/o): qemu extended regs
  194.       00000000fffc0000-00000000ffffffff (prio 0, rom): pc.bios
  195.     00000000000a0000-00000000000bffff (prio 1, i/o): alias smram-region @pci 00000000000a0000-00000000000bffff
  196.     00000000000c0000-00000000000c3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000c0000-00000000000c3fff
  197.     00000000000c4000-00000000000c7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000c4000-00000000000c7fff
  198.     00000000000c8000-00000000000cbfff (prio 1, ram): alias pam-rom @pc.ram 00000000000c8000-00000000000cbfff
  199.     00000000000cb000-00000000000cdfff (prio 1000, ram): alias kvmvapic-rom @pc.ram 00000000000cb000-00000000000cdfff
  200.     00000000000cc000-00000000000cffff (prio 1, ram): alias pam-rom @pc.ram 00000000000cc000-00000000000cffff
  201.     00000000000d0000-00000000000d3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000d0000-00000000000d3fff
  202.     00000000000d4000-00000000000d7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000d4000-00000000000d7fff
  203.     00000000000d8000-00000000000dbfff (prio 1, ram): alias pam-rom @pc.ram 00000000000d8000-00000000000dbfff
  204.     00000000000dc000-00000000000dffff (prio 1, ram): alias pam-rom @pc.ram 00000000000dc000-00000000000dffff
  205.     00000000000e0000-00000000000e3fff (prio 1, ram): alias pam-rom @pc.ram 00000000000e0000-00000000000e3fff
  206.     00000000000e4000-00000000000e7fff (prio 1, ram): alias pam-rom @pc.ram 00000000000e4000-00000000000e7fff
  207.     00000000000e8000-00000000000ebfff (prio 1, ram): alias pam-ram @pc.ram 00000000000e8000-00000000000ebfff
  208.     00000000000ec000-00000000000effff (prio 1, ram): alias pam-ram @pc.ram 00000000000ec000-00000000000effff
  209.     00000000000f0000-00000000000fffff (prio 1, ram): alias pam-rom @pc.ram 00000000000f0000-00000000000fffff
  210.     00000000fec00000-00000000fec00fff (prio 0, i/o): ioapic
  211.     00000000fed00000-00000000fed003ff (prio 0, i/o): hpet
  212.     00000000fee00000-00000000feefffff (prio 4096, i/o): apic-msi
  213.
  • address-space: memory

第一个是系统全局的AddressSpace，表示 虚拟机 能够访问的所有地址。

  • address-space: I/O

I/O表示x86系统下I/O端口的地址空间。

  • address-space: cpu-memory-0

cpu-memory-x表示CPU视角下的地址空间。

  • address-space: i440FX和address-space: PIIX3

i440FX和PIIX3是设备视角下的地址空间，虽然大部分情况下很多都是相同的，但是逻辑意义并不一样。

至此，QEMU中与内存相关的第一个数据结构AddressSpace就讲解完了。

（2）MemoryRegion

第二个是基本数据结构是MemoryRegion（就是上边AddressSpace中提到的）。MemoryRegion结构的定义在include/exec/memory.h中，如下： 

  1. #define TYPE_MEMORY_REGION "memory-region"
  2. DECLARE_INSTANCE_CHECKER(MemoryRegion, MEMORY_REGION,
  3.                          TYPE_MEMORY_REGION)

在此不详细展开宏，而直接给出struct MemoryRegion定义，也在include/exec/memory.h中，代码如下： 

  1. /** MemoryRegion:
  2.  *
  3.  * A struct representing a memory region.
  4.  */
  5. struct MemoryRegion {
  6.     Object parent_obj;
  7.  
  8.     /* private: */
  9.  
  10.     /* The following fields should fit in a cache line */
  11.     bool romd_mode;
  12.     bool ram;
  13.     bool subpage;
  14.     bool readonly; /* For RAM regions */
  15.     bool nonvolatile;
  16.     bool rom_device;
  17.     bool flush_coalesced_mmio;
  18.     uint8_t dirty_log_mask;
  19.     bool is_iommu;
  20.     RAMBlock *ram_block;
  21.     Object *owner;
  22.     /* owner as TYPE_DEVICE. Used for re-entrancy checks in MR access hotpath */
  23.     DeviceState *dev;
  24.  
  25.     const MemoryRegionOps *ops;
  26.     void *opaque;
  27.     MemoryRegion *container;
  28.     int mapped_via_alias; /* Mapped via an alias, container might be NULL */
  29.     Int128 size;
  30.     hwaddr addr;
  31.     void (*destructor)(MemoryRegion *mr);
  32.     uint64_t align;
  33.     bool terminates;
  34.     bool ram_device;
  35.     bool enabled;
  36.     bool warning_printed; /* For reservations */
  37.     uint8_t vga_logging_count;
  38.     MemoryRegion *alias;
  39.     hwaddr alias_offset;
  40.     int32_t priority;
  41.     QTAILQ_HEAD(, MemoryRegion) subregions;
  42.     QTAILQ_ENTRY(MemoryRegion) subregions_link;
  43.     QTAILQ_HEAD(, CoalescedMemoryRange) coalesced;
  44.     const char *name;
  45.     unsigned ioeventfd_nb;
  46.     MemoryRegionIoeventfd *ioeventfds;
  47.     RamDiscardManager *rdm; /* Only for RAM */
  48.  
  49.     /* For devices designed to perform re-entrant IO into their own IO MRs */
  50.     bool disable_reentrancy_guard;
  51. };

MemoryRegion表示的是虚拟机中的一段内存区域 （对比：AddressSpace结构用来表示一个虚拟机或者虚拟CPU能够访问的所有物理地址，其描述了一个到MemoryRegion对象的地址映射）。 MemoryRegion是内存模拟中的核心结构 ， 整个内存的模拟都是通过MemoryRegion构成的无环图完成的 。图的 叶子节点是实际分配给虚拟机的物理内存或者MMIO ， 中间节点则表示内存总线 ，内存控制器是其它MemoryRegion的别名。AddressSpace与MemoryRegion的关系如下图所示：

对于MemoryRegion各成员的详细讲解，请看下回。

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