hw/s390x/s390-pci-bus.c: Use g_assert_not_reached() in functions taking an ett

The s390-pci-bus.c code, Coverity complains about a possible overflow
because get_table_index() can return -1 if the ett value passed in is
not one of the three permitted ZPCI_ETT_PT, ZPCI_ETT_ST, ZPCI_ETT_RT,
but the caller in table_translate() doesn't check this and instead
uses the return value directly in a calculation of the guest address
to read from.

In fact this case cannot happen, because:
 * get_table_index() is called only from table_translate()
 * the only caller of table_translate() loops through the ett values
   in the order RT, ST, PT until table_translate() returns 0
 * table_translate() will return 0 for the error cases and when
   translate_iscomplete() returns true
 * translate_iscomplete() is always true for ZPCI_ETT_PT

So table_translate() is always called with a valid ett value.

Instead of having the various functions called from table_translate()
return a default or dummy value when the ett argument is out of range,
use g_assert_not_reached() to indicate that this is impossible.

Coverity: CID 1547609
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Message-ID: <20250710161552.1287399-1-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/s390x/s390-pci-bus.c

@@ -384,9 +384,9 @@ static uint64_t get_table_index(uint64_t iova, int8_t ett)
         return calc_sx(iova);
     case ZPCI_ETT_RT:
         return calc_rtx(iova);
+    default:
+        g_assert_not_reached();
     }
-
-    return -1;
 }
 
 static bool entry_isvalid(uint64_t entry, int8_t ett)
@@ -397,22 +397,24 @@ static bool entry_isvalid(uint64_t entry, int8_t ett)
     case ZPCI_ETT_ST:
     case ZPCI_ETT_RT:
         return rt_entry_isvalid(entry);
+    default:
+        g_assert_not_reached();
     }
-
-    return false;
 }
 
 /* Return true if address translation is done */
 static bool translate_iscomplete(uint64_t entry, int8_t ett)
 {
     switch (ett) {
-    case 0:
+    case ZPCI_ETT_ST:
         return (entry & ZPCI_TABLE_FC) ? true : false;
-    case 1:
+    case ZPCI_ETT_RT:
         return false;
+    case ZPCI_ETT_PT:
+        return true;
+    default:
+        g_assert_not_reached();
     }
-
-    return true;
 }
 
 static uint64_t get_frame_size(int8_t ett)
@@ -424,9 +426,9 @@ static uint64_t get_frame_size(int8_t ett)
         return 1ULL << 20;
     case ZPCI_ETT_RT:
         return 1ULL << 31;
+    default:
+        g_assert_not_reached();
     }
-
-    return 0;
 }
 
 static uint64_t get_next_table_origin(uint64_t entry, int8_t ett)
@@ -438,9 +440,9 @@ static uint64_t get_next_table_origin(uint64_t entry, int8_t ett)
         return get_st_pto(entry);
     case ZPCI_ETT_RT:
         return get_rt_sto(entry);
+    default:
+        g_assert_not_reached();
     }
-
-    return 0;
 }
 
 /**