-To compile the kernel module (which is required for operation), you
-must also install the following:
+ - libpcre, the Perl Compatible Regular Expression library, is
+ optional but recommended. Without it, OVSDB will not be able to
+ validate regular-expression based contraints.
+
+To compile the kernel module, you must also install the following. If
+you cannot build or install the kernel module, you may use the
+userspace-only implementation, at a cost in performance. The
+userspace implementation may also lack some features. Refer to
+INSTALL.userspace for more information.