x86: fix switch_to() clobbers
Liu Pingfan noticed that switch_to() clobbers more registers than its
asm constraints specify.
We get away with this due to luck mostly - schedule()
by its nature only has 'local' state which gets reloaded
automatically. Fix it nevertheless, we could hit this anytime.
it turns out that with the extra constraints gcc manages to make
schedule() even more compact:
text data bss dec hex filename
28626 684 2640 31950 7cce sched.o.before
28613 684 2640 31937 7cc1 sched.o.after
Reported-by: Liu Pingfan <kernelfans@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/asm-x86/system.h b/include/asm-x86/system.h
index 299ae96..33b0017 100644
--- a/include/asm-x86/system.h
+++ b/include/asm-x86/system.h
@@ -29,7 +29,14 @@
*/
#define switch_to(prev, next, last) \
do { \
- unsigned long esi, edi; \
+ /* \
+ * Context-switching clobbers all registers, so we clobber \
+ * them explicitly, via unused output variables. \
+ * (EAX and EBP is not listed because EBP is saved/restored \
+ * explicitly for wchan access and EAX is the return value of \
+ * __switch_to()) \
+ */ \
+ unsigned long ebx, ecx, edx, esi, edi; \
\
asm volatile( \
"pushfl \n\t" /* save flags */ \
@@ -49,6 +56,7 @@
"=a" (last), \
\
/* clobbered output registers: */ \
+ "=b" (ebx), "=c" (ecx), "=d" (edx), \
"=S" (esi), "=D" (edi) \
\
/* input parameters: */ \