s390/stack: add union to reflect kvm stack slot usages

Add a union which describes how the empty stack slots are being used
by kvm and perf. This should help to avoid another bug like the one
which was fixed with commit c9bfb460 ("s390/perf: obtain sie_block
from the right address").
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Tested-by: Nico Boehr <nrb@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/stacktrace.h

@@ -39,7 +39,15 @@ static inline bool on_stack(struct stack_info *info,
  * Kernel uses the packed stack layout (-mpacked-stack).
  */
 struct stack_frame {
-	unsigned long empty[9];
+	union {
+		unsigned long empty[9];
+		struct {
+			unsigned long sie_control_block;
+			unsigned long sie_savearea;
+			unsigned long sie_reason;
+			unsigned long sie_flags;
+		};
+	};
 	unsigned long gprs[10];
 	unsigned long back_chain;
 };

arch/s390/kernel/asm-offsets.c

@@ -58,10 +58,10 @@ int main(void)
 	OFFSET(__SF_BACKCHAIN, stack_frame, back_chain);
 	OFFSET(__SF_GPRS, stack_frame, gprs);
 	OFFSET(__SF_EMPTY, stack_frame, empty[0]);
-	OFFSET(__SF_SIE_CONTROL, stack_frame, empty[1]);
-	OFFSET(__SF_SIE_SAVEAREA, stack_frame, empty[2]);
-	OFFSET(__SF_SIE_REASON, stack_frame, empty[3]);
-	OFFSET(__SF_SIE_FLAGS, stack_frame, empty[4]);
+	OFFSET(__SF_SIE_CONTROL, stack_frame, sie_control_block);
+	OFFSET(__SF_SIE_SAVEAREA, stack_frame, sie_savearea);
+	OFFSET(__SF_SIE_REASON, stack_frame, sie_reason);
+	OFFSET(__SF_SIE_FLAGS, stack_frame, sie_flags);
 	DEFINE(STACK_FRAME_OVERHEAD, sizeof(struct stack_frame));
 	BLANK();
 	/* idle data offsets */

arch/s390/kernel/perf_event.c

@@ -30,7 +30,7 @@ static struct kvm_s390_sie_block *sie_block(struct pt_regs *regs)
 	if (!stack)
 		return NULL;
 
-	return (struct kvm_s390_sie_block *)stack->empty[1];
+	return (struct kvm_s390_sie_block *)stack->sie_control_block;
 }
 
 static bool is_in_guest(struct pt_regs *regs)