migration: Add VMSTATE_WITH_TMP

VMSTATE_WITH_TMP is for handling structures where some calculation
or rearrangement of the data needs to be performed before the data
hits the wire.
For example,  where the value on the wire is an offset from a
non-migrated base, but the data in the structure is the actual pointer.

To use it, a temporary type is created and a vmsd used on that type.
The first element of the type must be 'parent' a pointer back to the
type of the main structure.  VMSTATE_WITH_TMP takes care of allocating
and freeing the temporary before running the child vmsd.

The post_load/pre_save on the child vmsd can copy things from the parent
to the temporary using the parent pointer and do any other calculations
needed; it can then use normal VMSD entries to do the actual data
storage without having to fiddle around with qemu_get_*/qemu_put_*

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20170203160651.19917-3-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
index 7339594337..63e7b02e05 100644
--- a/include/migration/vmstate.h
+++ b/include/migration/vmstate.h
@@ -259,6 +259,7 @@ extern const VMStateInfo vmstate_info_cpudouble;
 extern const VMStateInfo vmstate_info_timer;
 extern const VMStateInfo vmstate_info_buffer;
 extern const VMStateInfo vmstate_info_unused_buffer;
+extern const VMStateInfo vmstate_info_tmp;
 extern const VMStateInfo vmstate_info_bitmap;
 extern const VMStateInfo vmstate_info_qtailq;
 
@@ -649,6 +650,24 @@ extern const VMStateInfo vmstate_info_qtailq;
     .offset     = offsetof(_state, _field),                          \
 }
 
+/* Allocate a temporary of type 'tmp_type', set tmp->parent to _state
+ * and execute the vmsd on the temporary.  Note that we're working with
+ * the whole of _state here, not a field within it.
+ * We compile time check that:
+ *    That _tmp_type contains a 'parent' member that's a pointer to the
+ *        '_state' type
+ *    That the pointer is right at the start of _tmp_type.
+ */
+#define VMSTATE_WITH_TMP(_state, _tmp_type, _vmsd) {                 \
+    .name         = "tmp",                                           \
+    .size         = sizeof(_tmp_type) +                              \
+                    QEMU_BUILD_BUG_ON_ZERO(offsetof(_tmp_type, parent) != 0) + \
+                    type_check_pointer(_state,                       \
+                        typeof_field(_tmp_type, parent)),            \
+    .vmsd         = &(_vmsd),                                        \
+    .info         = &vmstate_info_tmp,                               \
+}
+
 #define VMSTATE_UNUSED_BUFFER(_test, _version, _size) {              \
     .name         = "unused",                                        \
     .field_exists = (_test),                                         \