+  When the IPoIB driver is loaded, it creates one interface for each
+  port using the P_Key at index 0.  To create an interface with a
+  different P_Key, write the desired P_Key into the main interface's
+  /sys/class/net/<intf name>/create_child file.  For example:
+
+    echo 0x8001 > /sys/class/net/ib0/create_child
+
+  This will create an interface named ib0.8001 with P_Key 0x8001.  To
+  remove a subinterface, use the "delete_child" file:
+
+    echo 0x8001 > /sys/class/net/ib0/delete_child
+
+  The P_Key for any interface is given by the "pkey" file, and the
+  main interface for a subinterface is in "parent."

Any reason this doesn't use an interface similar to the normal vlan code?

And what is a P_Key?