linux内核已经实现了内存热插的支持，当一个linux系统不管运行在 物理环境 或者 虚拟环境 时只要宿主能提供内存热插拔机制，linux内核就能相应的增加或者减少内存。
目前已知的VMware已经可以做到内存热插。
改变内存容量有两种方式：
40 1.1 purpose of memory hotplug
41 ------------
42 Memory Hotplug allows users to increase/decrease the amount of memory.
43 Generally, there are two purposes.
44
45 (A) For changing the amount of memory.
46 This is to allow a feature like capacity on demand.
47 (B) For installing/removing DIMMs or NUMA-nodes physically.
48 This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.
前些天检查maillist时发现已经有人实现了DIMMs 方式更改内存大小。这种方式还没有并入mainline tree。
40 1.1 purpose of memory hotplug
41 ------------
42 Memory Hotplug allows users to increase/decrease the amount of memory.
43 Generally, there are two purposes.
44
45 (A) For changing the amount of memory.
46 This is to allow a feature like capacity on demand.
47 (B) For installing/removing DIMMs or NUMA-nodes physically.
48 This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.
可以测试下：
1）下载带有 dimm hotplug 功能的qemu-kvm：
git clone -b memhp-v3-alt https://github.com/vliaskov/qemu-kvm/commits/memhp-v3-alt
配置编译:
$./configure --target-list="i386-softmmu x86_64-softmmu i386-linux-user x86_64-linux-user" --enable-kvm
$make
2）下载linux内核 linux-3.0.1.tar.bz2 做测试：（32bit）
linux memory hotplug 需要内核编译进如下选项：
- For all memory hotplug
Memory model -> Sparse Memory (CONFIG_SPARSEMEM)
location:
Processor type and features
Memory model (Sparse Memory) --->
Allow for memory hot-add (CONFIG_MEMORY_HOTPLUG)
location:
Power management and ACPI options
ACPI (Advanced Configuration and Power Interface) Support
Memory Hotplug
- To enable memory removal, the followings are also necessary
Allow for memory hot remove (CONFIG_MEMORY_HOTREMOVE)
Page Migration (CONFIG_MIGRATION)
- For ACPI memory hotplug, the followings are also necessary
Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY)
This option can be kernel module.
- As a related configuration, if your box has a feature of NUMA-node hotplug
via ACPI, then this option is necessary too.
ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
(CONFIG_ACPI_CONTAINER).
This option can be kernel module too.
配置好之后就可以用make 了。
编译完成后将得到的bzImage文件拷贝出来。
3)制作启动所需固件：
当内存hotplugged之后，内核需要知道新增加上的内存并且制作内存管理表，生成sysfs文件来操作内存。
由于这需要固件的支持，而现有的qemu mainline 中包含的bios并不支持相应的功能，所以需要下载bios来重新编译：
git clone -b memhp-v3-alt https://github.com/vliaskov/seabios/commits/memhp-v3-alt
创建一个放固件的目录：bios
里面放的固件如下所示：（这些都是编译后qemu目录下的）
.
├── bios.bin
├── keymaps
│ ├── common
│ ├── en-us
│ └── modifiers
├── kvmvapic.bin
├── linuxboot.bin
├── multiboot.bin
├── optionrom
│ ├── kvmvapic.bin
│ ├── linuxboot.bin
│ └── multiboot.bin
├── ppc_rom.bin
├── pxe-e1000.rom
├── sgabios.bin
├── slof.bin
├── spapr-rtas.bin
├── vgabios.bin
├── vgabios-cirrus.bin
├── vgabios-qxl.bin
├── vgabios-stdvga.bin
└── vgabios-vmware.bin
将编译好的固件 ./out/bios.bin 替换上面的 bios.bin。
4）制作文件系统busybox.img
可参考如下连接：
http://blog.csdn.net/gudujianjsk/article/details/7655855
现在测试用的内核文件都得到了，用如下脚本开启测试：
./../qemu-kvm/i386-softmmu/qemu-system-i386 -enable-kvm -m 512 -kernel bzImage -L ./../bios -append "root=/dev/sda" -dimm id=dimm0,size=512M,node=0,populated=off -boot c -smp 1 -hda busybox.img -monitor stdio -k en-us -vnc :2
启动后系统显示如下：
由于编译时配置将 CONFIG_ACPI_HOTPLUG_MEMORY 配置为内核模块了，所以这里需要首先加载一个内核模块：acpi_memhotplug.ko.
所有的sections都在sysfs下有它们的设备信息。每一个section都是在 /sys/devices/system/memory/目录下的内存块的一部分。
/sys/devices/system/memory/memoryXXX
在这里XXX就是内存ID。
在monitor中动态 增加dimm：
[email protected]:/var/dbg/test-dimm/busybox$ sudo ./test-box.sh
QEMU 1.2.50 monitor - type 'help' for more information
(qemu) device_add dimm,id=dimm0
此时在虚拟机中可见多了个memory1，其状态是offline：
此时新增加的内存还没有做好使用的准备。为了使用新增加的内存你需要把新增加的内存设置为online 状态
此时用free查看总计的内存数量：
测试完毕。
http://www.mjmwired.net/kernel/Documentation/memory-hotplug.txt