vmstate: add VMS_MUST_EXIST

Can be used to verify a required field exists or validate
state in some other way.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

include/migration/vmstate.h

@@ -100,6 +100,7 @@ enum VMStateFlags {
     VMS_MULTIPLY         = 0x200,  /* multiply "size" field by field_size */
     VMS_VARRAY_UINT8     = 0x400,  /* Array with size in uint8_t field*/
     VMS_VARRAY_UINT32    = 0x800,  /* Array with size in uint32_t field*/
+    VMS_MUST_EXIST       = 0x1000, /* Field must exist in input */
 };
 
 typedef struct {

vmstate.c

@@ -102,6 +102,10 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
                     return ret;
                 }
             }
+        } else if (field->flags & VMS_MUST_EXIST) {
+            fprintf(stderr, "Input validation failed: %s/%s\n",
+                    vmsd->name, field->name);
+            return -1;
         }
         field++;
     }
@@ -142,6 +146,12 @@ void vmstate_save_state(QEMUFile *f, const VMStateDescription *vmsd,
                     field->info->put(f, addr, size);
                 }
             }
+        } else {
+            if (field->flags & VMS_MUST_EXIST) {
+                fprintf(stderr, "Output state validation failed: %s/%s\n",
+                        vmsd->name, field->name);
+                assert(!(field->flags & VMS_MUST_EXIST));
+            }
         }
         field++;
     }