migration: Add VMS_NO_STATE flag

There are a few special cases of vmstate usage:

The vmstate_msix and vmstate_scsi_device have fields that contain no
data, only a vmstate_info structure.

The VMSTATE_VALIDATE macro serves only to invoke the .field_exists
routine for validation.

Regardless whether these scenarios are valid, add a separate flag to
identify them so we can enforce common constraints for the normal
vmstates such as having a size greater than zero.

Note that n_elems is hardcoded to 1 for all vmstates, except
VMS_[V]ARRAY, so VMSTATE_VALIDATE needed to set VMS_ARRAY to be able
to force n_elems to 0. This patch now checks the flag at
vmstate_n_elems().

Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

1 files changed, 5 insertions, 1 deletions

diff --git a/migration/vmstate.c b/migration/vmstate.c
index 9556030f89..1f9cb45923 100644
--- a/migration/vmstate.c
+++ b/migration/vmstate.c
@@ -110,12 +110,16 @@ static uint64_t vmstate_read_from_offset(const VMStateStructMember *member,
 
 static uint64_t vmstate_n_elems(void *opaque, const VMStateField *field)
 {
-    uint64_t n_elems = 1;
+    uint64_t n_elems;
 
     if (field->flags & VMS_ARRAY) {
         n_elems = field->num;
     } else if (field->flags & VMS_VARRAY) {
         n_elems = vmstate_read_from_offset(&field->num_indirect, opaque);
+    } else if (field->flags & VMS_MUST_EXIST && field->flags & VMS_NO_STATE) {
+        n_elems = 0;
+    } else {
+        n_elems = 1;
     }
 
     trace_vmstate_n_elems(field->name, n_elems);