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QEMU下的内存结构MemoryRegion和AddressSpace

Virtualization 2017-06-15

QEMU下的内存结构体很多了,RAMBlock,MemoryRegion,AddressSpace,MemoryRegionSection,KVMSlot和kvm_userspace_memory_region,很多时候看代码时候都会被搅合成一坨,虽然很早时候在KVM源代码分析2:虚拟机的创建与运行KVM源代码分析4:内存虚拟化都提到了内存虚拟化的过程,但实际上这几个结构体之间的关系并没有理清,外加上lizhen童鞋要求分析讲一讲,故在此处补个补丁。

QEMU代码:git v2.8.0

RAMBLOCK才是真正分配了host内存的地方,如果把它直接理解成一个内存条也是非常合适的,但实际上不仅仅如此,还有设备自有内存,显存。它的主要元素就是mr,host, offset和used_length。

struct RAMBlock {
 struct rcu_head rcu;
 struct MemoryRegion *mr;
 uint8_t *host;
 ram_addr_t offset;
 ram_addr_t used_length;
 ram_addr_t max_length;
 void (*resized)(const char*, uint64_t length, void *host);
 uint32_t flags;
 /* Protected by iothread lock. */
 char idstr[256];
 /* RCU-enabled, writes protected by the ramlist lock */
 QLIST_ENTRY(RAMBlock) next;
 int fd;
 size_t page_size;
};

每个RAMBLOCK都有一个唯一的MemoryRegion对应,另外需要注意的是不是每个MemoryRegion都有RAMBLOCK对应的。host则是host的线性地址

 new_block->host = phys_mem_alloc(new_block->max_length, &new_block->mr->align);

offset则是这个block在ram中的base地址,如qemu_get_ram_block下的代码

block = atomic_rcu_read(&ram_list.mru_block);
 if (block && addr - block->offset < block->max_length) {
 return block;
 }

offset则是block在ram中的偏移位置

然后看MemoryRegion

struct MemoryRegion {
 Object parent_obj;

/* All fields are private - violators will be prosecuted */

/* The following fields should fit in a cache line */
 bool romd_mode;
 bool ram;
 bool subpage;
 bool readonly; /* For RAM regions */
 bool rom_device;
 bool flush_coalesced_mmio;
 bool global_locking;
 uint8_t dirty_log_mask;
 RAMBlock *ram_block;
 Object *owner;
 const MemoryRegionIOMMUOps *iommu_ops;

const MemoryRegionOps *ops;
 void *opaque;
 MemoryRegion *container;
 Int128 size;
 hwaddr addr;
 void (*destructor)(MemoryRegion *mr);
 uint64_t align;
 bool terminates;
 bool ram_device;
 bool enabled;
 bool warning_printed; /* For reservations */
 uint8_t vga_logging_count;
 MemoryRegion *alias;
 hwaddr alias_offset;
 int32_t priority;
 QTAILQ_HEAD(subregions, MemoryRegion) subregions;
 QTAILQ_ENTRY(MemoryRegion) subregions_link;
 QTAILQ_HEAD(coalesced_ranges, CoalescedMemoryRange) coalesced;
 const char *name;
 unsigned ioeventfd_nb;
 MemoryRegionIoeventfd *ioeventfds;
 QLIST_HEAD(, IOMMUNotifier) iommu_notify;
 IOMMUNotifierFlag iommu_notify_flags;
};

MemoryRegion是树状父子结构的,每一个ramblock都有一个对应的MemoryRegion,一般而言,这个MemoryRegion是最顶级的MemoryRegion,它还有很多子MemoryRegion,比如在这个ramblock地址范围内的MMIO等,这里面的重点元素是size,addr和alias_offset,

memory_region_init_alias(ram_below_4g, NULL, "ram-below-4g", ram, 0, pcms->below_4g_mem_size);

memory_region_init(mr, owner, name, size);

mr->size = int128_make64(size);

mr->alias_offset = offset;

addr的由来则是

memory_region_add_subregion(system_memory, 0x100000000ULL,ram_above_4g);

memory_region_add_subregion_common(mr, offset, subregion);

subregion->addr = offset;

此处的addr实际也是mr的起始地址,和offset有点类似

struct AddressSpace {
 /* All fields are private. */
 struct rcu_head rcu;
 char *name;
 MemoryRegion *root;
 int ref_count;
 bool malloced;

/* Accessed via RCU. */
 struct FlatView *current_map;

int ioeventfd_nb;
 struct MemoryRegionIoeventfd *ioeventfds;
 struct AddressSpaceDispatch *dispatch;
 struct AddressSpaceDispatch *next_dispatch;
 MemoryListener dispatch_listener;
 QTAILQ_HEAD(memory_listeners_as, MemoryListener) listeners;
 QTAILQ_ENTRY(AddressSpace) address_spaces_link;
};

地址空间,本来不同的设备使用的地址空间不同,但是QEMU X86里面只有两种,address_space_memory和address_space_io,所有设备的地址空间都被映射到了这两个上面,

struct FlatRange {
 MemoryRegion *mr;
 hwaddr offset_in_region;
 AddrRange addr;
 uint8_t dirty_log_mask;
 bool romd_mode;
 bool readonly;
};

struct FlatView {
 struct rcu_head rcu;
 unsigned ref;
 FlatRange *ranges;
 unsigned nr;
 unsigned nr_allocated;
};

是通过FlatView体现的,当memory region发生变化的时候,执行memory_region_transaction_commit,address_space_update_topology,address_space_update_topology_pass最终完成更新FlatView的目标。

FlatView结构如下,图片来自刘峰童鞋。

每一个flat range都投射到同一个地址空间的平面上,而上图中的R1,R2等对应的则是struct MemoryRegionSection。

struct MemoryRegionSection {
 MemoryRegion *mr;
 AddressSpace *address_space;
 hwaddr offset_within_region;
 Int128 size;
 hwaddr offset_within_address_space;
 bool readonly;
};

在下面被调用

#define MEMORY_LISTENER_UPDATE_REGION(fr, as, dir, callback, _args...) \
 do { \
 MemoryRegionSection mrs = section_from_flat_range(fr, as); \
 MEMORY_LISTENER_CALL(as, callback, dir, &mrs, ##_args); \
 } while(0)

在MEMORY_LISTENER_CALL中调用kvm_region_add和kvm_region_del,执行kvm_set_phys_mem,组装KVMSlot

typedef struct KVMSlot
{
 hwaddr start_addr;
 ram_addr_t memory_size;
 void *ram;
 int slot;
 int flags;
} KVMSlot;

最终通过kvm_userspace_memory_region将QEMU的内存分布信息传递给KVM。

具体元素的关系,见下图

 


QEMU下的内存结构MemoryRegion和AddressSpace来自于OenHan,链接为:http://oenhan.com/qemu-memory-struct
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