diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
index 717f2621e2..0c5e33c78f 100644
--- a/docs/formatdomain.rst
+++ b/docs/formatdomain.rst
@@ -1928,6 +1928,9 @@ are:
    ``custom``
       Set exactly the specified features.
 
+   ``passthrough``
+      Enable all features currently supported by the hypervisor.
+
    The ``mode`` attribute can be omitted and will default to ``custom``.
 
 ``pvspinlock``