s390/mm: allow to check if a gmap shadow is valid

It will be very helpful to have a mechanism to check without any locks
if a given gmap shadow is still valid and matches the given properties.
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

arch/s390/include/asm/gmap.h

@@ -109,6 +109,7 @@ int gmap_read_table(struct gmap *gmap, unsigned long gaddr, unsigned long *val);
 
 struct gmap *gmap_shadow(struct gmap *parent, unsigned long asce,
 			 int edat_level);
+int gmap_shadow_valid(struct gmap *sg, unsigned long asce, int edat_level);
 int gmap_shadow_r2t(struct gmap *sg, unsigned long saddr, unsigned long r2t,
 		    int fake);
 int gmap_shadow_r3t(struct gmap *sg, unsigned long saddr, unsigned long r3t,

arch/s390/mm/gmap.c

@@ -1407,6 +1407,26 @@ static struct gmap *gmap_find_shadow(struct gmap *parent, unsigned long asce,
 	return NULL;
 }
 
+/**
+ * gmap_shadow_valid - check if a shadow guest address space matches the
+ *                     given properties and is still valid
+ * @sg: pointer to the shadow guest address space structure
+ * @asce: ASCE for which the shadow table is requested
+ * @edat_level: edat level to be used for the shadow translation
+ *
+ * Returns 1 if the gmap shadow is still valid and matches the given
+ * properties, the caller can continue using it. Returns 0 otherwise, the
+ * caller has to request a new shadow gmap in this case.
+ *
+ */
+int gmap_shadow_valid(struct gmap *sg, unsigned long asce, int edat_level)
+{
+	if (sg->removed)
+		return 0;
+	return sg->orig_asce == asce && sg->edat_level == edat_level;
+}
+EXPORT_SYMBOL_GPL(gmap_shadow_valid);
+
 /**
  * gmap_shadow - create/find a shadow guest address space
  * @parent: pointer to the parent gmap