接前一篇文章： QEMU源码全解析 —— 内存虚拟化（16）
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
本回继续对于QEMU内存平坦化的核心函数 —— render_memory_region()进行深入解析。为了便于理解和回顾，再次贴出render_memory_region函数代码，在softmmu/memory.c中，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);
}
}上一回讲到了以下代码片段：
/* Render the region itself into any gaps left by the current view. */
for (i = 0; i < view->nr && int128_nz(remain); ++i) {
……
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);
}
……
}1）计算now，即上一回图中（见下边）左边深色区域的长度。
now = int128_min(remain,
int128_sub(view->ranges[i].addr.start, base));2）之后设置fr.offset_in_region为在mr2中的偏移（此偏移为0）。
offset_in_region = int128_get64(int128_sub(clip.start, base));
……
/* Render the region itself into any gaps left by the current view. */
for (i = 0; i < view->nr && int128_nz(remain); ++i) {
……
fr.offset_in_region = offset_in_region;
……3）fr.addr描述的是该FlatRange在虚拟机物理地址空间中的位置，也就是左边深色区域的起始位置。
fr.addr = addrrange_make(base, now);4）接着调用flatview_insert函数，将这个fr插入到view中。
flatview_insert(view, i, &fr);5）然后更新base，此时base跟fr2的开始位置一样。
int128_addto(&base, now);6）接下来，更新remain，剩下fr2+右边的深色区域。
offset_in_region += int128_get64(now);
int128_subfrom(&remain, now);
接下来是for循环中的以下代码片段：
/* Render the region itself into any gaps left by the current view. */
for (i = 0; i < view->nr && int128_nz(remain); ++i) {
……
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);
}对于for循环中该段代码以及render_memory_region函数后续代码的解析，请看下回。
