s390/mem_detect: fix tprot() program check new psw handling

The tprot() inline asm temporarily changes the program check new psw
to redirect a potential program check on the diag instruction.
Restoring of the program check new psw is done in C code behind the
inline asm.

This can be problematic, especially if the function is inlined, since
the compiler can reorder instructions in such a way that a different
instruction, which may result in a program check, might be executed
before the program check new psw has been restored.

To avoid such a scenario move restoring into the inline asm. For
consistency reasons move also saving of the original program check new
psw into the inline asm.

Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>

arch/s390/boot/mem_detect.c

···
 
 static int tprot(unsigned long addr)
 {
-	unsigned long pgm_addr;
+	unsigned long reg1, reg2;
 	int rc = -EFAULT;
-	psw_t old = S390_lowcore.program_new_psw;
+	psw_t old;
 
-	S390_lowcore.program_new_psw.mask = __extract_psw();
 	asm volatile(
-		"	larl	%[pgm_addr],1f\n"
-		"	stg	%[pgm_addr],%[psw_pgm_addr]\n"
+		"	mvc	0(16,%[psw_old]),0(%[psw_pgm])\n"
+		"	epsw	%[reg1],%[reg2]\n"
+		"	st	%[reg1],0(%[psw_pgm])\n"
+		"	st	%[reg2],4(%[psw_pgm])\n"
+		"	larl	%[reg1],1f\n"
+		"	stg	%[reg1],8(%[psw_pgm])\n"
 		"	tprot	0(%[addr]),0\n"
 		"	ipm	%[rc]\n"
 		"	srl	%[rc],28\n"
-		"1:\n"
-		: [pgm_addr] "=&d"(pgm_addr),
-		  [psw_pgm_addr] "=Q"(S390_lowcore.program_new_psw.addr),
-		  [rc] "+&d"(rc)
-		: [addr] "a"(addr)
+		"1:	mvc	0(16,%[psw_pgm]),0(%[psw_old])\n"
+		: [reg1] "=&d" (reg1),
+		  [reg2] "=&a" (reg2),
+		  [rc] "+&d" (rc),
+		  "=Q" (S390_lowcore.program_new_psw.addr),
+		  "=Q" (old)
+		: [psw_old] "a" (&old),
+		  [psw_pgm] "a" (&S390_lowcore.program_new_psw),
+		  [addr] "a" (addr)
 		: "cc", "memory");
-	S390_lowcore.program_new_psw = old;
 	return rc;
 }