Linux进程（之）进程切换函数switch_to()解析

进程切换过程是由一段汇编代码switch_to完成，主要功能是保存当前进程的esp和eip，恢复切换到的进程的esp和eip。
关于C内嵌汇编的使用，可以参见：http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

其调用过程如下：

switch_to(prev, next, prev);barrier();finish_task_switch(this_rq(), prev);

从上面代码可以看出，保证进程被切换回来后，prev是一个正确值也是switch_to需要考虑的问题。
如： A=>B, B=>C, C=>A
若不进行处理时，在进程A被切换出去，再切换回来后，prev指向的是A。
而期望的值为C，这样才能对C做进一步的清理工作。

swith_to代码如下：

#define switch_to(prev,next,last) do {\unsigned long esi,edi;\asm volatile("pushfl\n\t"/* Save flags */\     "pushl %%ebp\n\t"\     "movl %%esp,%0\n\t"/* save ESP */\     "movl %5,%%esp\n\t"/* restore ESP */\     "movl $1f,%1\n\t"/* save EIP */\     "pushl %6\n\t"/* restore EIP */\     "jmp __switch_to\n"\     "1:\t"\     "popl %%ebp\n\t"\     "popfl"\     :"=m" (prev->thread.esp),"=m" (prev->thread.eip),\      "=a" (last),"=S" (esi),"=D" (edi)\     :"m" (next->thread.esp),"m" (next->thread.eip),\      "2" (prev), "d" (next));\} while (0)

下面对A=>B, B=>C, C=>A的切换过程中，进程A的相关处理过程进行分析：

A=>B : switch_to(A, B, A)1. pushfl2. pushl %%ebp   pushl %ebp3. movl %%esp,%0   movl %esp (A->thread.esp)4. movl $1f,%1   movl $1f, (A->thread.eip)C=>A: switch_to(C, A,  C)5. movl %5,%%esp   movl (A->thread.esp) %esp6. pushl %6   pushl (A->thread.eip)7. jmp __switch_to   __switch_to(%eax(C), %edx(A))   ret #popl %eip(A->thread.eip <=> $1f)8. popl %%ebp   popl %ebp9. popfl10. "=m" (prev->thread.esp),"=m" (prev->thread.eip)    movl %esp, (A->thread.esp)    movl %eip, (A->thread.eip)11. "=a" (last),"=S" (esi),"=D" (edi)    movl %eax(C), (last)(<=>prev)    movl %esi, (esi)    movl %edi, (edi)

注意这里的10和11两步：
在A的栈中，prev与next的值是在A=>B时赋予的，所以prev=A, next=B；
10这条语句更新的是A的thread相关字段
11这条语句执行后， prev=last=C，好做进一步处理

疑点：关于esi, edi两个变量的作用，还没有弄明白！

整理后的切换流程如下：

A=>Bpushflpushl %ebpmovl %esp (A->thread.esp)movl $1f, (A->thread.eip)C=>Amovl (A->thread.esp) %esppushl (A->thread.eip)call __switch_to(C, A)popl %eippopl %ebppopflmovl %esp (A->thread.esp)movl %eip (A->thread.eip)movl (C) (last)