今天在网上找到一篇文档，描述了SLES12版本上Xen的一些变化：
（1）grub2
SLES12上使用grub2配置Xen参数。以配置dom0_max_vcpus为例：
a）SLES11使用/boot/grub/menu.lst（参考这里）；
b）SLES12使用/etc/default/grub（参考这里）。
（2）xm/xend已经废弃了，要使用xl/libxl。下图做了一个简单对比：
（3）SLES12 Xen Stack：
• Xen 4.4.1
• kernel 3.12.x
• libvirt 1.2.5
• virt-install 1.1.x, vm-install 1.x.x
• virt-manager 1.1.x
为什么会发生memory reorder？一言以蔽之，因为性能。
在支持memory reorder的系统上，有以下3种order需要考虑：
Program order: the order in which the memory operations are specified in the code running on a given CPU.
Execution order: the order in which the individual memory-reference instructions are executed on a given CPU. The execution order can differ from program order due to both compiler and CPU-implementation optimizations.
Perceived order: the order in which a given CPU perceives its and other CPUs’ memory operations. The perceived order can differ from the execution order due to caching, interconnect and memory-system optimizations. Different CPUs might well perceive the same memory operations as occurring in different orders.
Program order是代码里访问内存的顺序。Execution order是代码在CPU上实际执行的顺序，由于编译器优化和CPU的实现，实际指令执行的顺序有可能和代码顺序不一样。Perceived order是CPU用来“感知”自己或者其它CPU对内存操作，由于caching，interconnect等原因，这个顺序有可能与代码实际的execution order不同。
关于memory order的总结：
A given CPU always perceives its own memory operations as occurring in program order. That is, memory-reordering issues arise only when a CPU is observing other CPUs’ memory operations.
An operation is reordered with a store only if the operation accesses a different location than does the store.
Aligned simple loads and stores are atomic.
Linux-kernel synchronization primitives contain any needed memory barriers, which is a good reason to use these primitives.
参考资料：
Memory Reordering Caught in the Act；
Memory Ordering in Modern Microprocessors, Part I。
