接前一篇文章： QEMU源码全解析 —— 内存虚拟化（12）
本文内容参考：
《 QEMU /KVM源码解析与应用》 —— 李强，机械工业出版社
浅谈QEMU Memory Region 与 Address Space
QEMU内存分析（一）：内存虚拟化关键结构体 - Edver - 博客园
特此致谢！
2. QEMU虚拟机内存初始化
上一回重点讲解了FlatRange的定义以及FlatView与FlatRange的关系，本回开始对于生成FlatView的函数generate_memory_topology进行解析。为了便于理解和回顾，再次贴出generate_memory_topology函数和其调用者address_space_update_topology函数的源码，两个均在softmmu/memory.c中，代码如下：
static void address_space_update_topology(AddressSpace *as)
{
MemoryRegion *physmr = memory_region_get_flatview_root(as->root);
flatviews_init();
if (!g_hash_table_lookup(flat_views, physmr)) {
generate_memory_topology(physmr);
}
address_space_set_flatview(as);
}/* Render a memory topology into a list of disjoint absolute ranges. */
static FlatView *generate_memory_topology(MemoryRegion *mr)
{
int i;
FlatView *view;
view = flatview_new(mr);
if (mr) {
render_memory_region(view, mr, int128_zero(),
addrrange_make(int128_zero(), int128_2_64()),
false, false);
}
flatview_simplify(view);
view->dispatch = address_space_dispatch_new(view);
for (i = 0; i < view->nr; i++) {
MemoryRegionSection mrs =
section_from_flat_range(&view->ranges[i], view);
flatview_add_to_dispatch(view, &mrs);
}
address_space_dispatch_compact(view->dispatch);
g_hash_table_replace(flat_views, mr, view);
return view;
}
首先概述一下generate_memory_topology函数的整体功能。generate_memory_topology函数总共有两个功能：
（1）第一个功能是调用render_memory_region函数，将一个MemoryRegion展开，并把数据记录到一个FlatView中。render_memory_region函数会递归调用其下的子Region，直到遇到叶子节点为止。
（2）第二个功能是调用flatview_simplify函数（simplify的意义为简化），将FlatView中能够合并的FlatRange进行合并，从而减少FlatRange的个数。
接下来具体解析generate_memory_topology函数的实现细节。
generate_memory_topology函数首先调用flatview_new函数，创建并初始化一个空的FlatView。代码片段如下：
FlatView *view;
view = flatview_new(mr);flatview_new函数也在softmmu/memory.c中，代码如下：
static FlatView *flatview_new(MemoryRegion *mr_root)
{
FlatView *view;
view = g_new0(FlatView, 1);
view->ref = 1;
view->root = mr_root;
memory_region_ref(mr_root);
trace_flatview_new(view, mr_root);
return view;
}memory_region_ref函数也在同文件中，代码如下：
void memory_region_ref(MemoryRegion *mr)
{
/* MMIO callbacks most likely will access data that belongs
* to the owner, hence the need to ref/unref the owner whenever
* the memory region is in use.
*
* The memory region is a child of its owner. As long as the
* owner doesn't call unparent itself on the memory region,
* ref-ing the owner will also keep the memory region alive.
* Memory regions without an owner are supposed to never go away;
* we do not ref/unref them because it slows down DMA sensibly.
*/
if (mr && mr->owner) {
object_ref(mr->owner);
}
}接下来，generate_memory_topology函数调用render_memory_region函数，进行树状MemoryRegion的展开。其中涉及到了128位整数的计算，这里不深入细节了，可以将其作为一个普通整数来理解。
render_memory_region函数也在softmmu/memory.c中，代码如下：
/* Render a memory region into the global view. Ranges in @view obscure
* ranges in @mr.
*/
static void render_memory_region(FlatView *view,
MemoryRegion *mr,
Int128 base,
AddrRange clip,
bool readonly,
bool nonvolatile)
{
MemoryRegion *subregion;
unsigned i;
hwaddr offset_in_region;
Int128 remain;
Int128 now;
FlatRange fr;
AddrRange tmp;
if (!mr->enabled) {
return;
}
int128_addto(&base, int128_make64(mr->addr));
readonly |= mr->readonly;
nonvolatile |= mr->nonvolatile;
tmp = addrrange_make(base, mr->size);
if (!addrrange_intersects(tmp, clip)) {
return;
}
clip = addrrange_intersection(tmp, clip);
if (mr->alias) {
int128_subfrom(&base, int128_make64(mr->alias->addr));
int128_subfrom(&base, int128_make64(mr->alias_offset));
render_memory_region(view, mr->alias, base, clip,
readonly, nonvolatile);
return;
}
/* Render subregions in priority order. */
QTAILQ_FOREACH(subregion, &mr->subregions, subregions_link) {
render_memory_region(view, subregion, base, clip,
readonly, nonvolatile);
}
if (!mr->terminates) {
return;
}
offset_in_region = int128_get64(int128_sub(clip.start, base));
base = clip.start;
remain = clip.size;
fr.mr = mr;
fr.dirty_log_mask = memory_region_get_dirty_log_mask(mr);
fr.romd_mode = mr->romd_mode;
fr.readonly = readonly;
fr.nonvolatile = nonvolatile;
/* Render the region itself into any gaps left by the current view. */
for (i = 0; i < view->nr && int128_nz(remain); ++i) {
if (int128_ge(base, addrrange_end(view->ranges[i].addr))) {
continue;
}
if (int128_lt(base, view->ranges[i].addr.start)) {
now = int128_min(remain,
int128_sub(view->ranges[i].addr.start, base));
fr.offset_in_region = offset_in_region;
fr.addr = addrrange_make(base, now);
flatview_insert(view, i, &fr);
++i;
int128_addto(&base, now);
offset_in_region += int128_get64(now);
int128_subfrom(&remain, now);
}
now = int128_sub(int128_min(int128_add(base, remain),
addrrange_end(view->ranges[i].addr)),
base);
int128_addto(&base, now);
offset_in_region += int128_get64(now);
int128_subfrom(&remain, now);
}
if (int128_nz(remain)) {
fr.offset_in_region = offset_in_region;
fr.addr = addrrange_make(base, remain);
flatview_insert(view, i, &fr);
}
}render_memory_region函数是QEMU内存平坦化的核心函数，下一回对其进行详细解析。
