s390/sclp: make early sclp irq handler more robust

Make the early sclp interrupt handler more robust:

- disable all interrupt sub classes except for the service signal subclass
- extend ctlreg0 union so it is easily possible to set the service signal
  subclass mask bit without using a magic number
- disable lowcore protection before writing to it
- make sure that all write accesses are done before the original content
  of control register 0 is restored, which could enable lowcore protection

Reviewed-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/ctl_reg.h

@@ -62,7 +62,9 @@ union ctlreg0 {
 		unsigned long	   : 4;
 		unsigned long afp  : 1; /* AFP-register control */
 		unsigned long vx   : 1; /* Vector enablement control */
-		unsigned long	   : 17;
+		unsigned long	   : 7;
+		unsigned long sssm : 1; /* Service signal subclass mask */
+		unsigned long	   : 9;
 	};
 };
 

arch/s390/kernel/sclp.c

@@ -18,12 +18,16 @@ static bool have_linemode __section(data);
 
 static void _sclp_wait_int(void)
 {
-	unsigned long cr0, cr0_new, psw_mask, addr;
+	unsigned long psw_mask, addr, flags;
 	psw_t psw_ext_save, psw_wait;
+	union ctlreg0 cr0, cr0_new;
 
-	__ctl_store(cr0, 0, 0);
-	cr0_new = cr0 | 0x200;
-	__ctl_load(cr0_new, 0, 0);
+	raw_local_irq_save(flags);
+	__ctl_store(cr0.val, 0, 0);
+	cr0_new.val = cr0.val & ~CR0_IRQ_SUBCLASS_MASK;
+	cr0_new.lap = 0;
+	cr0_new.sssm = 1;
+	__ctl_load(cr0_new.val, 0, 0);
 
 	psw_ext_save = S390_lowcore.external_new_psw;
 	psw_mask = __extract_psw();
@@ -45,8 +49,9 @@ static void _sclp_wait_int(void)
 			: "cc", "memory");
 	} while (S390_lowcore.ext_int_code != EXT_IRQ_SERVICE_SIG);
 
-	__ctl_load(cr0, 0, 0);
 	S390_lowcore.external_new_psw = psw_ext_save;
+	__ctl_load(cr0.val, 0, 0);
+	raw_local_irq_restore(flags);
 }
 
 static int _sclp_servc(unsigned int cmd, char *sccb)