diff --git a/sys/amd64/amd64/machdep.c b/sys/amd64/amd64/machdep.c
index cc2b581b624..14cb1373d12 100644
--- a/sys/amd64/amd64/machdep.c
+++ b/sys/amd64/amd64/machdep.c
@@ -1147,7 +1147,7 @@ struct soft_segment_descriptor gdt_segs[] = {
 	.ssd_gran = 1		},
 /* GPROC0_SEL	9 Proc 0 Tss Descriptor */
 {	.ssd_base = 0x0,
-	.ssd_limit = sizeof(struct amd64tss) + IOPAGES * PAGE_SIZE - 1,
+	.ssd_limit = sizeof(struct amd64tss) + IOPERM_BITMAP_SIZE - 1,
 	.ssd_type = SDT_SYSTSS,
 	.ssd_dpl = SEL_KPL,
 	.ssd_p = 1,
@@ -2003,8 +2003,7 @@ hammer_time(u_int64_t modulep, u_int64_t physfree)
 	common_tss[0].tss_ist2 = (long) np;
 
 	/* Set the IO permission bitmap (empty due to tss seg limit) */
-	common_tss[0].tss_iobase = sizeof(struct amd64tss) +
-	    IOPAGES * PAGE_SIZE;
+	common_tss[0].tss_iobase = sizeof(struct amd64tss) + IOPERM_BITMAP_SIZE;
 
 	gsel_tss = GSEL(GPROC0_SEL, SEL_KPL);
 	ltr(gsel_tss);
diff --git a/sys/amd64/amd64/mp_machdep.c b/sys/amd64/amd64/mp_machdep.c
index 484fc061e00..6e59645c2bd 100644
--- a/sys/amd64/amd64/mp_machdep.c
+++ b/sys/amd64/amd64/mp_machdep.c
@@ -637,7 +637,7 @@ init_secondary(void)
 	common_tss[cpu] = common_tss[0];
 	common_tss[cpu].tss_rsp0 = 0;   /* not used until after switch */
 	common_tss[cpu].tss_iobase = sizeof(struct amd64tss) +
-	    IOPAGES * PAGE_SIZE;
+	    IOPERM_BITMAP_SIZE;
 	common_tss[cpu].tss_ist1 = (long)&doublefault_stack[PAGE_SIZE];
 
 	/* The NMI stack runs on IST2. */
diff --git a/sys/amd64/amd64/sys_machdep.c b/sys/amd64/amd64/sys_machdep.c
index 26f38a234ac..1d9bd4cbb4e 100644
--- a/sys/amd64/amd64/sys_machdep.c
+++ b/sys/amd64/amd64/sys_machdep.c
@@ -338,7 +338,6 @@ amd64_set_ioperm(td, uap)
 	char *iomap;
 	struct amd64tss *tssp;
 	struct system_segment_descriptor *tss_sd;
-	u_long *addr;
 	struct pcb *pcb;
 
 	if ((error = priv_check(td, PRIV_IO)) != 0)
@@ -361,9 +360,7 @@ amd64_set_ioperm(td, uap)
 		if (tssp == NULL)
 			return (ENOMEM);
 		iomap = (char *)&tssp[1];
-		addr = (u_long *)iomap;
-		for (i = 0; i < (ctob(IOPAGES) + 1) / sizeof(u_long); i++)
-			*addr++ = ~0;
+		memset(iomap, 0xff, IOPERM_BITMAP_SIZE);
 		critical_enter();
 		/* Takes care of tss_rsp0. */
 		memcpy(tssp, &common_tss[PCPU_GET(cpuid)],
diff --git a/sys/amd64/include/param.h b/sys/amd64/include/param.h
index 3d4722b1f0c..7dcd7dd7e64 100644
--- a/sys/amd64/include/param.h
+++ b/sys/amd64/include/param.h
@@ -120,6 +120,12 @@
 #define	MAXPAGESIZES	3	/* maximum number of supported page sizes */
 
 #define IOPAGES	2		/* pages of i/o permission bitmap */
+/*
+ * I/O permission bitmap has a bit for each I/O port plus an additional
+ * byte at the end with all bits set. See section "I/O Permission Bit Map"
+ * in the Intel SDM for more details.
+ */
+#define	IOPERM_BITMAP_SIZE	(IOPAGES * PAGE_SIZE + 1)
 
 #ifndef	KSTACK_PAGES
 #define	KSTACK_PAGES	4	/* pages of kstack (with pcb) */