switch_to宏的实现:
27 /*
28 * Saving eflags is important. It switches not only IOPL between tasks,
29 * it also protects other tasks from NT leaking through sysenter etc.
30 */
31 #define switch_to(prev, next, last) \
32 do { \
33 /* \
34 * Context-switching clobbers all registers, so we clobber \
35 * them explicitly, via unused output variables. \
36 * (EAX and EBP is not listed because EBP is saved/restored \
37 * explicitly for wchan access and EAX is the return value of \
38 * __switch_to()) \
39 */ \
40 unsigned long ebx, ecx, edx, esi, edi; \
41 \
42 asm volatile("pushfl\n\t" /* save flags */ \
43 "pushl %%ebp\n\t" /* save EBP */ \
44 "movl %%esp,%[prev_sp]\n\t" /* save ESP */ \
45 "movl %[next_sp],%%esp\n\t" /* restore ESP */ \
46 "movl $1f,%[prev_ip]\n\t" /* save EIP */ \
47 "pushl %[next_ip]\n\t" /* restore EIP */ \
48 __switch_canary \
49 "jmp __switch_to\n" /* regparm call */ \
50 "1:\t" \
51 "popl %%ebp\n\t" /* restore EBP */ \
52 "popfl\n" /* restore flags */ \
53 \
54 /* output parameters */ \
55 : [prev_sp] "=m" (prev->thread.sp), \
56 [prev_ip] "=m" (prev->thread.ip), \
57 "=a" (last), \
58 \
59 /* clobbered output registers: */ \
60 "=b" (ebx), "=c" (ecx), "=d" (edx), \
61 "=S" (esi), "=D" (edi) \
62 \
63 __switch_canary_oparam \
64 \
65 /* input parameters: */ \
66 : [next_sp] "m" (next->thread.sp), \
67 [next_ip] "m" (next->thread.ip), \
68 \
69 /* regparm parameters for __switch_to(): */ \
70 [prev] "a" (prev), \
71 [next] "d" (next) \
72 \
73 __switch_canary_iparam \
74 \
75 : /* reloaded segment registers */ \
76 "memory"); \
77 } while (0)关键代码：
44 "movl %%esp,%[prev_sp]\n\t"其中，prev_sp的定义为：
55 : [prev_sp] "=m" (prev->thread.sp)也即把当前堆栈的值保存到当前进程的task_struct结构的thread结构体中，而
45 "movl %[next_sp],%%esp\n\t"则把保存在要被切换到的进程的task_struct结构的thread结构体中的堆栈值，赋值给esp寄存器，即栈顶指针指向了要切换到的进程的内核栈，实现了堆栈的切换。
而
47 "pushl %[next_ip]\n\t"
49 "jmp __switch_to\n"则实现了cpu执行流的切换，即切换到next执行指令。47行把next_ip，即next->thread.ip的值压入堆栈，然后跳转到__switch_to处执行，由于__switch_to是一个函数，并且是使用的jmp实现的跳转，这样会把刚刚压入堆栈的next->thread.ip值当成返回地址。因此在该函数返回(ret)的时候，cpu会跳转到next->thread.ip指向的指令流执行，从而实现进程切换。
关于寄存器的保存：
该宏的实现中，通过下面的语句，通用寄存器ebx, ecx, edx, esi, edi的值被故意修改了：
60 "=b" (ebx), "=c" (ecx), "=d" (edx),
61 "=S" (esi), "=D" (edi)即不保存和恢复通用寄存器的值，而只保存了和恢复标志寄存器，堆栈寄存器等。因为是在内核态进行切换，所以cs和ds的值无需修改(都是指向内核代码段和数据段)。
那为什么在用户空间到内核空间的切换过程中，通用寄存器的值需要被保存呢？个人以为，因为切换的时候，可能通过寄存器还在使用当中，所以需要进行保存。