diff --git a/sys/amd64/include/pmap.h b/sys/amd64/include/pmap.h
index ef02ac110075..c362aa60451a 100644
--- a/sys/amd64/include/pmap.h
+++ b/sys/amd64/include/pmap.h
@@ -114,10 +114,10 @@
  */
 #define	PML4PML4I	(NPML4EPG/2)	/* Index of recursive pml4 mapping */
 
-#define	KPML4I		(NPML4EPG-1)
-#define DMPML4I		(KPML4I-1)
+#define	KPML4I		(NPML4EPG-1)	/* Top 512GB for KVM */
+#define	DMPML4I		(KPML4I-1)	/* Next 512GB down for direct map */
 
-#define	KPDPI		(NPDPEPG-1)
+#define	KPDPI		(NPDPEPG-1)	/* kernbase at -1GB */
 
 /*
  * XXX doesn't really belong here I guess...