Index: x86/x86/busdma_machdep.c
===================================================================
--- x86/x86/busdma_machdep.c	(revision 232207)
+++ x86/x86/busdma_machdep.c	(working copy)
@@ -674,6 +674,14 @@
 	vaddr = (vm_offset_t)buf;
 	lastaddr = *lastaddrp;
 	bmask = ~(dmat->boundary - 1);
+#if !defined(__i386__) || defined(PAE)
+	/*
+	 * Always enforce at least a 4GB boundary.  Note that this is not
+	 * needed on non-PAE i386.
+	 */
+	if (dmat->boundary == 0 || dmat->boundary > ((bus_addr_t)1 << 32))
+		bmask = ~(((bus_addr_t)1 << 32) - 1);
+#endif
 
 	for (seg = *segp; buflen > 0 ; ) {
 		bus_size_t max_sgsize;