s390/uaccess: introduce 'uaccesspt' kernel parameter

The uaccesspt kernel parameter allows to enforce using the uaccess page
table walk variant. This is mainly for debugging purposes, so this mode
can also be enabled on machines which support the mvcos instruction.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/lib/uaccess_mvcos.c

@@ -241,9 +241,22 @@ long __strncpy_from_user(char *dst, const char __user *src, long count)
 }
 EXPORT_SYMBOL(__strncpy_from_user);
 
+/*
+ * The uaccess page tabe walk variant can be enforced with the "uaccesspt"
+ * kernel parameter. This is mainly for debugging purposes.
+ */
+static int force_uaccess_pt __initdata;
+
+static int __init parse_uaccess_pt(char *__unused)
+{
+	force_uaccess_pt = 1;
+	return 0;
+}
+early_param("uaccesspt", parse_uaccess_pt);
+
 static int __init uaccess_init(void)
 {
-	if (IS_ENABLED(CONFIG_32BIT) || !test_facility(27))
+	if (IS_ENABLED(CONFIG_32BIT) || force_uaccess_pt || !test_facility(27))
 		static_key_slow_dec(&have_mvcos);
 	return 0;
 }