virtio-mem: Fail if a memory backend with "prealloc=on" is specified

"prealloc=on" for the memory backend does not work as expected, as
virtio-mem will simply discard all preallocated memory immediately again.
In the best case, it's an expensive NOP. In the worst case, it's an
unexpected allocation error.

Instead, "prealloc=on" should be specified for the virtio-mem device only,
such that virtio-mem will try preallocating memory before plugging
memory dynamically to the guest. Fail if such a memory backend is
provided.

Tested-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>S
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index 1ed1f5a..02f7b54 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -772,6 +772,12 @@
         error_setg(errp, "'%s' property specifies an unsupported memdev",
                    VIRTIO_MEM_MEMDEV_PROP);
         return;
+    } else if (vmem->memdev->prealloc) {
+        error_setg(errp, "'%s' property specifies a memdev with preallocation"
+                   " enabled: %s. Instead, specify 'prealloc=on' for the"
+                   " virtio-mem device. ", VIRTIO_MEM_MEMDEV_PROP,
+                   object_get_canonical_path_component(OBJECT(vmem->memdev)));
+        return;
     }
 
     if ((nb_numa_nodes && vmem->node >= nb_numa_nodes) ||