QEMU源码全解析 —— 内存虚拟化（4）

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本文深入讲解QEMU内存管理中的RAMBlock结构，它表示虚拟机的内存条，记录内存条的基本信息，如物理内存大小、文件后端、内存偏移等。RAMBlock与MemoryRegion的关系以及在内存模拟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（3）

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

浅谈QEMU Memory Region 与 Address Space

特此致谢！

QEMU 内存初始化

1. 基本结构

上一回对于QEMU中与内存相关的第二个数据结构MemoryRegion进行了深入讲解。本回讲解第三个数据结构RAMBlock。

（3）RAMBlock

内存管理最基础的一部分自然是物理Memory内存，然后还包括MMIO空间、IO端口的地址空间。RAMBlock结构表示的是内存条，一个RAMBlock对应 虚拟机 中的一个内存条。RAMBlock结构的定义在include/qemu/typedefs.h中，代码如下： 

typedef struct RAMBlock RAMBlock;

struct RAMBlock的定义在include/exec/ramblock.h中，代码如下： 

  1. struct RAMBlock {
  2.     struct rcu_head rcu;
  3.     struct MemoryRegion *mr;
  4.     uint8_t *host;
  5.     uint8_t *colo_cache; /* For colo, VM's ram cache */
  6.     ram_addr_t offset;
  7.     ram_addr_t used_length;
  8.     ram_addr_t max_length;
  9.     void (*resized)(const char*, uint64_t length, void *host);
  10.     uint32_t flags;
  11.     /* Protected by iothread lock.  */
  12.     char idstr[256];
  13.     /* RCU-enabled, writes protected by the ramlist lock */
  14.     QLIST_ENTRY(RAMBlock) next;
  15.     QLIST_HEAD(, RAMBlockNotifier) ramblock_notifiers;
  16.     int fd;
  17.     uint64_t fd_offset;
  18.     size_t page_size;
  19.     /* dirty bitmap used during migration */
  20.     unsigned long *bmap;
  21.     /* bitmap of already received pages in postcopy */
  22.     unsigned long *receivedmap;
  23.  
  24.     /*
  25.      * bitmap to track already cleared dirty bitmap.  When the bit is
  26.      * set, it means the corresponding memory chunk needs a log-clear.
  27.      * Set this up to non-NULL to enable the capability to postpone
  28.      * and split clearing of dirty bitmap on the remote node (e.g.,
  29.      * KVM).  The bitmap will be set only when doing global sync.
  30.      *
  31.      * It is only used during src side of ram migration, and it is
  32.      * protected by the global ram_state.bitmap_mutex.
  33.      *
  34.      * NOTE: this bitmap is different comparing to the other bitmaps
  35.      * in that one bit can represent multiple guest pages (which is
  36.      * decided by the `clear_bmap_shift' variable below).  On
  37.      * destination side, this should always be NULL, and the variable
  38.      * `clear_bmap_shift' is meaningless.
  39.      */
  40.     unsigned long *clear_bmap;
  41.     uint8_t clear_bmap_shift;
  42.  
  43.     /*
  44.      * RAM block length that corresponds to the used_length on the migration
  45.      * source (after RAM block sizes were synchronized). Especially, after
  46.      * starting to run the guest, used_length and postcopy_length can differ.
  47.      * Used to register/unregister uffd handlers and as the size of the received
  48.      * bitmap. Receiving any page beyond this length will bail out, as it
  49.      * could not have been valid on the source.
  50.      */
  51.     ram_addr_t postcopy_length;
  52. };

上边已提到，RAMBlock结构表示的是虚拟机中的内存条，一个RAMBlock对应虚拟机中的一个内存条。RAMBlock里面记录了该内存条的一些基本信息，如所属的mr（struct MemoryRegion *mr）、如果有文件作为后端，该文件对应的fd（int fd）、系统的页面大小page_size（size_t page_size）、已经使用的大小used_length（ram_addr_t used_length）、该内存条在虚拟机整个内存中的偏移offset（ram_addr_t offset）等。

每个MemoryRegion里都包含一个RAMBlock的指针，但不一定会对应一个RAMBlock。对于物理内存，则其实体MemoryRegion会指向一个实体RAMBlock。回顾一下MemoryRegion结构中的RAMBlock的相关成员，在include/exec/memory.h中，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.     ……
  22. };
  • RAMBlock *ram_block

ram_block表示实际分配的物理内存。

回到struct RAMBlock的定义。其中的主线逻辑变量offset（ram_addr_t offset）（GPA）和host（uint8_t *host）（HVA）。还有bmap（unsigned long *bmap）和receivedmap（unsigned long *receivedmap）是热迁移存储脏页使用。

此外，所有的RAMBlock会通过next（QLIST_ENTRY(RAMBlock) next）域连接到一个链表中，链表头是ram_list.blocks全局变量。

这里顺便提一下struct RAMBlock中ram_addr_t类型的定义。ram_addr_t的定义在include/exec/cpu-common.h中，代码如下： 

  1. /* address in the RAM (different from a physical address) */
  2. #if defined(CONFIG_XEN_BACKEND)
  3. typedef uint64_t ram_addr_t;
  4. #  define RAM_ADDR_MAX UINT64_MAX
  5. #  define RAM_ADDR_FMT "%" PRIx64
  6. #else
  7. typedef uintptr_t ram_addr_t;
  8. #  define RAM_ADDR_MAX UINTPTR_MAX
  9. #  define RAM_ADDR_FMT "%" PRIxPTR
  10. #endif

uint64_t和uintptr_t都在roms/opensbi/include/sbi/sbi_types.h中定义，分别如下： 

  1. #if __riscv_xlen == 64
  2. typedef long			s64;
  3. typedef unsigned long		u64;
  4. typedef long			int64_t;
  5. typedef unsigned long		uint64_t;
  6. #define PRILX			"016lx"
  7. #elif __riscv_xlen == 32
  8. typedef long long		s64;
  9. typedef unsigned long long	u64;
  10. typedef long long		int64_t;
  11. typedef unsigned long long	uint64_t;
  12. #define PRILX			"08lx"
  13. #else
  14. #error "Unexpected __riscv_xlen"
  15. #endif
typedef unsigned long		uintptr_t;

至此，QEMU中与内存相关的三个基本结构struct AddressSpace、struct MemoryRegion、struct RAMBlock就讲解完了。

再来回顾和复习一下这三个基本数据结构：

  • AddressSpace（struct AddressSpace）

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。struct AddressSpace的定义在include/exec/memory.h中，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. };
  • MemoryRegion（struct MemoryRegion）

MemoryRegion表示的是虚拟机中的一段内存区域。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，内存控制是其它MemoryRegion的别名。

struct MemoryRegion的定义也在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. };
  • RAMBlock（struct RAMBlock）

RAMBlock结构表示的是虚拟机中的内存条，一个RAMBlock对应虚拟机中的一个内存条。RAMBlock里面记录了该内存条的一些基本信息。struct RAMBlock的定义在include/exec/ramblock.h中，如下： 

  1. struct RAMBlock {
  2.     struct rcu_head rcu;
  3.     struct MemoryRegion *mr;
  4.     uint8_t *host;
  5.     uint8_t *colo_cache; /* For colo, VM's ram cache */
  6.     ram_addr_t offset;
  7.     ram_addr_t used_length;
  8.     ram_addr_t max_length;
  9.     void (*resized)(const char*, uint64_t length, void *host);
  10.     uint32_t flags;
  11.     /* Protected by iothread lock.  */
  12.     char idstr[256];
  13.     /* RCU-enabled, writes protected by the ramlist lock */
  14.     QLIST_ENTRY(RAMBlock) next;
  15.     QLIST_HEAD(, RAMBlockNotifier) ramblock_notifiers;
  16.     int fd;
  17.     uint64_t fd_offset;
  18.     size_t page_size;
  19.     /* dirty bitmap used during migration */
  20.     unsigned long *bmap;
  21.     /* bitmap of already received pages in postcopy */
  22.     unsigned long *receivedmap;
  23.  
  24.     /*
  25.      * bitmap to track already cleared dirty bitmap.  When the bit is
  26.      * set, it means the corresponding memory chunk needs a log-clear.
  27.      * Set this up to non-NULL to enable the capability to postpone
  28.      * and split clearing of dirty bitmap on the remote node (e.g.,
  29.      * KVM).  The bitmap will be set only when doing global sync.
  30.      *
  31.      * It is only used during src side of ram migration, and it is
  32.      * protected by the global ram_state.bitmap_mutex.
  33.      *
  34.      * NOTE: this bitmap is different comparing to the other bitmaps
  35.      * in that one bit can represent multiple guest pages (which is
  36.      * decided by the `clear_bmap_shift' variable below).  On
  37.      * destination side, this should always be NULL, and the variable
  38.      * `clear_bmap_shift' is meaningless.
  39.      */
  40.     unsigned long *clear_bmap;
  41.     uint8_t clear_bmap_shift;
  42.  
  43.     /*
  44.      * RAM block length that corresponds to the used_length on the migration
  45.      * source (after RAM block sizes were synchronized). Especially, after
  46.      * starting to run the guest, used_length and postcopy_length can differ.
  47.      * Used to register/unregister uffd handlers and as the size of the received
  48.      * bitmap. Receiving any page beyond this length will bail out, as it
  49.      * could not have been valid on the source.
  50.      */
  51.     ram_addr_t postcopy_length;
  52. };

39eea3608c2790fa6ea1027a7aaeefbe.png

基础已经打好，下一回开始讲解其中更为详细的内容。

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