diff --git a/target/s390x/diag.c b/target/s390x/diag.c
index 53c2f81f2a..b5aec06d6b 100644
--- a/target/s390x/diag.c
+++ b/target/s390x/diag.c
@@ -53,6 +53,29 @@ int handle_diag_288(CPUS390XState *env, uint64_t r1, uint64_t r3)
 #define DIAG_308_RC_NO_CONF         0x0102
 #define DIAG_308_RC_INVALID         0x0402
 
+#define DIAG308_RESET_MOD_CLR       0
+#define DIAG308_RESET_LOAD_NORM     1
+#define DIAG308_LOAD_CLEAR          3
+#define DIAG308_LOAD_NORMAL_DUMP    4
+#define DIAG308_SET                 5
+#define DIAG308_STORE               6
+
+static int diag308_parm_check(CPUS390XState *env, uint64_t r1, uint64_t addr,
+                              uintptr_t ra, bool write)
+{
+    if ((r1 & 1) || (addr & ~TARGET_PAGE_MASK)) {
+        s390_program_interrupt(env, PGM_SPECIFICATION, ra);
+        return -1;
+    }
+    if (!address_space_access_valid(&address_space_memory, addr,
+                                    sizeof(IplParameterBlock), write,
+                                    MEMTXATTRS_UNSPECIFIED)) {
+        s390_program_interrupt(env, PGM_ADDRESSING, ra);
+        return -1;
+    }
+    return 0;
+}
+
 void handle_diag_308(CPUS390XState *env, uint64_t r1, uint64_t r3, uintptr_t ra)
 {
     CPUState *cs = env_cpu(env);
@@ -65,30 +88,24 @@ void handle_diag_308(CPUS390XState *env, uint64_t r1, uint64_t r3, uintptr_t ra)
         return;
     }
 
-    if ((subcode & ~0x0ffffULL) || (subcode > 6)) {
+    if (subcode & ~0x0ffffULL) {
         s390_program_interrupt(env, PGM_SPECIFICATION, ra);
         return;
     }
 
     switch (subcode) {
-    case 0:
+    case DIAG308_RESET_MOD_CLR:
         s390_ipl_reset_request(cs, S390_RESET_MODIFIED_CLEAR);
         break;
-    case 1:
+    case DIAG308_RESET_LOAD_NORM:
         s390_ipl_reset_request(cs, S390_RESET_LOAD_NORMAL);
         break;
-    case 3:
+    case DIAG308_LOAD_CLEAR:
+        /* Well we still lack the clearing bit... */
         s390_ipl_reset_request(cs, S390_RESET_REIPL);
         break;
-    case 5:
-        if ((r1 & 1) || (addr & 0x0fffULL)) {
-            s390_program_interrupt(env, PGM_SPECIFICATION, ra);
-            return;
-        }
-        if (!address_space_access_valid(&address_space_memory, addr,
-                                        sizeof(IplParameterBlock), false,
-                                        MEMTXATTRS_UNSPECIFIED)) {
-            s390_program_interrupt(env, PGM_ADDRESSING, ra);
+    case DIAG308_SET:
+        if (diag308_parm_check(env, r1, addr, ra, false)) {
             return;
         }
         iplb = g_new0(IplParameterBlock, 1);
@@ -110,15 +127,8 @@ void handle_diag_308(CPUS390XState *env, uint64_t r1, uint64_t r3, uintptr_t ra)
 out:
         g_free(iplb);
         return;
-    case 6:
-        if ((r1 & 1) || (addr & 0x0fffULL)) {
-            s390_program_interrupt(env, PGM_SPECIFICATION, ra);
-            return;
-        }
-        if (!address_space_access_valid(&address_space_memory, addr,
-                                        sizeof(IplParameterBlock), true,
-                                        MEMTXATTRS_UNSPECIFIED)) {
-            s390_program_interrupt(env, PGM_ADDRESSING, ra);
+    case DIAG308_STORE:
+        if (diag308_parm_check(env, r1, addr, ra, true)) {
             return;
         }
         iplb = s390_ipl_get_iplb();