[S390] switch_to: dont restore/save access & fpu regs for kernel threads
If the previous task was a kernel thread there is no need to save the
contents of the fpu and access registers since they aren't used in
kernel mode.
For the same reason it is not necessary to restore these registers if
the next task is a kernel thread.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/include/asm/system.h b/arch/s390/include/asm/system.h
index 1f2ebc4..c35d0a3 100644
--- a/arch/s390/include/asm/system.h
+++ b/arch/s390/include/asm/system.h
@@ -85,14 +85,18 @@
asm volatile("lam 0,15,%0" : : "Q" (*acrs));
}
-#define switch_to(prev,next,last) do { \
- if (prev == next) \
- break; \
- save_fp_regs(&prev->thread.fp_regs); \
- restore_fp_regs(&next->thread.fp_regs); \
- save_access_regs(&prev->thread.acrs[0]); \
- restore_access_regs(&next->thread.acrs[0]); \
- prev = __switch_to(prev,next); \
+#define switch_to(prev,next,last) do { \
+ if (prev == next) \
+ break; \
+ if (prev->mm) { \
+ save_fp_regs(&prev->thread.fp_regs); \
+ save_access_regs(&prev->thread.acrs[0]); \
+ } \
+ if (next->mm) { \
+ restore_fp_regs(&next->thread.fp_regs); \
+ restore_access_regs(&next->thread.acrs[0]); \
+ } \
+ prev = __switch_to(prev,next); \
} while (0)
extern void account_vtime(struct task_struct *, struct task_struct *);