diff --git a/sys/riscv/include/pmap.h b/sys/riscv/include/pmap.h
--- a/sys/riscv/include/pmap.h
+++ b/sys/riscv/include/pmap.h
@@ -137,6 +137,13 @@
 #define	L1_MAPPABLE_P(va, pa, size)					\
 	((((va) | (pa)) & L1_OFFSET) == 0 && (size) >= L1_SIZE)
 
+enum pmap_mode {
+	PMAP_MODE_SV39,
+	PMAP_MODE_SV48,
+};
+
+extern enum pmap_mode pmap_mode;
+
 struct thread;
 
 #define	pmap_vm_page_alloc_check(m)
diff --git a/sys/riscv/include/pte.h b/sys/riscv/include/pte.h
--- a/sys/riscv/include/pte.h
+++ b/sys/riscv/include/pte.h
@@ -44,22 +44,24 @@
 typedef	uint64_t	pn_t;			/* page number */
 #endif
 
-/* Level 0 table, 512GiB per entry */
+/* Level 0 table, 512GiB per entry, SV48 only */
 #define	L0_SHIFT	39
+#define	L0_SIZE		(1UL << L0_SHIFT)
+#define	L0_OFFSET	(L0_SIZE - 1)
 
 /* Level 1 table, 1GiB per entry */
 #define	L1_SHIFT	30
-#define	L1_SIZE 	(1 << L1_SHIFT)
+#define	L1_SIZE 	(1UL << L1_SHIFT)
 #define	L1_OFFSET 	(L1_SIZE - 1)
 
 /* Level 2 table, 2MiB per entry */
 #define	L2_SHIFT	21
-#define	L2_SIZE 	(1 << L2_SHIFT)
+#define	L2_SIZE 	(1UL << L2_SHIFT)
 #define	L2_OFFSET 	(L2_SIZE - 1)
 
 /* Level 3 table, 4KiB per entry */
 #define	L3_SHIFT	12
-#define	L3_SIZE 	(1 << L3_SHIFT)
+#define	L3_SIZE 	(1UL << L3_SHIFT)
 #define	L3_OFFSET 	(L3_SIZE - 1)
 
 #define	Ln_ENTRIES_SHIFT 9
diff --git a/sys/riscv/include/vmparam.h b/sys/riscv/include/vmparam.h
--- a/sys/riscv/include/vmparam.h
+++ b/sys/riscv/include/vmparam.h
@@ -180,8 +180,10 @@
 	((va) - DMAP_MIN_ADDRESS) + dmap_phys_base;			\
 })
 
-#define	VM_MIN_USER_ADDRESS	(0x0000000000000000UL)
-#define	VM_MAX_USER_ADDRESS	(0x0000004000000000UL)
+#define	VM_MIN_USER_ADDRESS_SV39	(0x0000000000000000UL)
+#define	VM_MAX_USER_ADDRESS_SV39	(0x0000004000000000UL)
+#define	VM_MIN_USER_ADDRESS		VM_MIN_USER_ADDRESS_SV39
+#define	VM_MAX_USER_ADDRESS		VM_MAX_USER_ADDRESS_SV39
 
 #define	VM_MINUSER_ADDRESS	(VM_MIN_USER_ADDRESS)
 #define	VM_MAXUSER_ADDRESS	(VM_MAX_USER_ADDRESS)
@@ -191,8 +193,10 @@
     (((va) < VM_MAX_USER_ADDRESS) || ((va) >= VM_MIN_KERNEL_ADDRESS))
 
 #define	KERNBASE		(VM_MIN_KERNEL_ADDRESS)
-#define	SHAREDPAGE		(VM_MAXUSER_ADDRESS - PAGE_SIZE)
-#define	USRSTACK		SHAREDPAGE
+#define	SHAREDPAGE_SV39		(VM_MAX_USER_ADDRESS_SV39 - PAGE_SIZE)
+#define	SHAREDPAGE		SHAREDPAGE_SV39
+#define	USRSTACK		SHAREDPAGE_SV39
+#define	PS_STRINGS_SV39		(USRSTACK_SV39 - sizeof(struct ps_strings))
 
 #define	VM_EARLY_DTB_ADDRESS	(VM_MAX_KERNEL_ADDRESS - (2 * L2_SIZE))
 
diff --git a/sys/riscv/riscv/pmap.c b/sys/riscv/riscv/pmap.c
--- a/sys/riscv/riscv/pmap.c
+++ b/sys/riscv/riscv/pmap.c
@@ -160,7 +160,19 @@
 #include <machine/pcb.h>
 #include <machine/sbi.h>
 
-#define	NUL1E		(Ln_ENTRIES * Ln_ENTRIES)
+/*
+ * Boundary values for the page table page index space:
+ *
+ * L3 pages: [0, NUL2E)
+ * L2 pages: [NUL2E, NUL2E + NUL1E)
+ * L1 pages: [NUL2E + NUL1E, NUL2E + NUL1E + NUL0E)
+ *
+ * Note that these ranges are used in both SV39 and SV48 mode.  In SV39 mode the
+ * ranges are not fully populated since there are at most Ln_ENTRIES^2 L3 pages
+ * in a set of page tables.
+ */
+#define	NUL0E		Ln_ENTRIES
+#define	NUL1E		(Ln_ENTRIES * NUL0E)
 #define	NUL2E		(Ln_ENTRIES * NUL1E)
 
 #if !defined(DIAGNOSTIC)
@@ -179,6 +191,7 @@
 #define PV_STAT(x)	do { } while (0)
 #endif
 
+#define	pmap_l1_pindex(v)	(NUL2E + ((v) >> L1_SHIFT))
 #define	pmap_l2_pindex(v)	((v) >> L2_SHIFT)
 #define	pa_to_pvh(pa)		(&pv_table[pa_index(pa)])
 
@@ -219,6 +232,8 @@
 LIST_HEAD(pmaplist, pmap);
 static struct pmaplist allpmaps = LIST_HEAD_INITIALIZER();
 
+enum pmap_mode __read_frequently pmap_mode = PMAP_MODE_SV39;
+
 struct pmap kernel_pmap_store;
 
 vm_offset_t virtual_avail;	/* VA of first avail page (after kernel bss) */
@@ -338,6 +353,7 @@
 	bzero(p, PAGE_SIZE);
 }
 
+#define	pmap_l0_index(va)	(((va) >> L0_SHIFT) & Ln_ADDR_MASK)
 #define	pmap_l1_index(va)	(((va) >> L1_SHIFT) & Ln_ADDR_MASK)
 #define	pmap_l2_index(va)	(((va) >> L2_SHIFT) & Ln_ADDR_MASK)
 #define	pmap_l3_index(va)	(((va) >> L3_SHIFT) & Ln_ADDR_MASK)