x86: clean up switch_to()

Make the code more readable and more hackable:

 - use symbolic asm parameters
 - use readable indentation
 - add comments that explains the details

No code changed:

kernel/sched.o:

   text	   data	    bss	    dec	    hex	filename
  28626	    684	   2640	  31950	   7cce	sched.o.before
  28626	    684	   2640	  31950	   7cce	sched.o.after

md5:
   2823d406c18b781975cdb2e7cfea0059  sched.o.before.asm
   2823d406c18b781975cdb2e7cfea0059  sched.o.after.asm

Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/include/asm-x86/system.h b/include/asm-x86/system.h
index 428d9471497fe8d679cc7679c0c6bbfd66f8ca0d..299ae9605cb1631f2a3f871213f2455f5173dd3e 100644 (file)
--- a/include/asm-x86/system.h
+++ b/include/asm-x86/system.h
@@ -27,22 +27,38 @@ struct task_struct *__switch_to(struct task_struct *prev,
  * Saving eflags is important. It switches not only IOPL between tasks,
  * it also protects other tasks from NT leaking through sysenter etc.
  */
-#define switch_to(prev, next, last) do {                               \
+#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.sp), "=m" (prev->thread.ip),   \
-                     "=a" (last), "=S" (esi), "=D" (edi)               \
-                    :"m" (next->thread.sp), "m" (next->thread.ip),     \
-                     "2" (prev), "d" (next));                          \
+                                                                       \
+       asm volatile(                                                   \
+               "pushfl                 \n\t"   /* save    flags */     \
+               "pushl %%ebp            \n\t"   /* save    EBP   */     \
+               "movl %%esp,%[prev_sp]  \n\t"   /* save    ESP   */     \
+               "movl %[next_sp],%%esp  \n\t"   /* restore ESP   */     \
+               "movl $1f,%[prev_ip]    \n\t"   /* save    EIP   */     \
+               "pushl %[next_ip]       \n\t"   /* restore EIP   */     \
+               "jmp __switch_to        \n"     /* regparm call  */     \
+               "1:                     \t"                             \
+               "popl %%ebp             \n\t"   /* restore EBP   */     \
+               "popfl                  \n"     /* restore flags */     \
+                                                                       \
+                 /* output parameters */                               \
+               : [prev_sp] "=m" (prev->thread.sp),                     \
+                 [prev_ip] "=m" (prev->thread.ip),                     \
+                           "=a" (last),                                \
+                                                                       \
+                 /* clobbered output registers: */                     \
+                 "=S" (esi), "=D" (edi)                                \
+                                                                       \
+                 /* input parameters: */                               \
+               : [next_sp]  "m" (next->thread.sp),                     \
+                 [next_ip]  "m" (next->thread.ip),                     \
+                                                                       \
+                 /* regparm parameters for __switch_to(): */           \
+                 [prev]     "a" (prev),                                \
+                 [next]     "d" (next)                                 \
+       );                                                              \
 } while (0)
 
 /*