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>

1 files changed, 31 insertions, 15 deletions

diff --git a/include/asm-x86/system.h b/include/asm-x86/system.h
index 428d9471497f..299ae9605cb1 100644
--- 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)
 
 /*