migration: comment VMSTATE_UNUSED*() properly

It is error prone to use VMSTATE_UNUSED*() sometimes especially when
the size of the migration stream of the field is not the same as the
size of the structure (boolean is one example).  Comment it well so
people will be aware of this when people want to use it.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20190329095713.14177-1-peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
index a668ec75b83912d79aa8b8b109f62c895cf05743..9224370ed59a36324ea18f88424db656bb05208e 100644 (file)
--- a/include/migration/vmstate.h
+++ b/include/migration/vmstate.h
@@ -1035,6 +1035,20 @@ extern const VMStateInfo vmstate_info_qtailq;
 #define VMSTATE_BUFFER_UNSAFE(_field, _state, _version, _size)        \
     VMSTATE_BUFFER_UNSAFE_INFO(_field, _state, _version, vmstate_info_buffer, _size)
 
+/*
+ * These VMSTATE_UNUSED*() macros can be used to fill in the holes
+ * when some of the vmstate fields are obsolete to be compatible with
+ * migrations between new/old binaries.
+ *
+ * CAUTION: when using any of the VMSTATE_UNUSED*() macros please be
+ * sure that the size passed in is the size that was actually *sent*
+ * rather than the size of the *structure*.  One example is the
+ * boolean type - the size of the structure can vary depending on the
+ * definition of boolean, however the size we actually sent is always
+ * 1 byte (please refer to implementation of VMSTATE_BOOL_V and
+ * vmstate_info_bool).  So here we should always pass in size==1
+ * rather than size==sizeof(bool).
+ */
 #define VMSTATE_UNUSED_V(_v, _size)                                   \
     VMSTATE_UNUSED_BUFFER(NULL, _v, _size)