diff --git a/sys/amd64/amd64/db_trace.c b/sys/amd64/amd64/db_trace.c
index bd737dd28baa..880be61a5839 100644
--- a/sys/amd64/amd64/db_trace.c
+++ b/sys/amd64/amd64/db_trace.c
@@ -562,7 +562,7 @@ amd64_set_watch(watchnum, watchaddr, size, access, d)
 	}
 
 	/*
-	 * we can watch a 1, 2, or 4 byte sized location
+	 * we can watch a 1, 2, 4, or 8 byte sized location
 	 */
 	switch (size) {
 	case 1:
@@ -574,6 +574,9 @@ amd64_set_watch(watchnum, watchaddr, size, access, d)
 	case 4:
 		len = DBREG_DR7_LEN_4;
 		break;
+	case 8:
+		len = DBREG_DR7_LEN_8;
+		break;
 	default:
 		return (-1);
 	}
@@ -624,12 +627,14 @@ db_md_set_watchpoint(addr, size)
 			avail++;
 	}
 
-	if (avail * 4 < size)
+	if (avail * 8 < size)
 		return (-1);
 
 	for (i = 0; i < 4 && (size > 0); i++) {
 		if (!DBREG_DR7_ENABLED(d.dr[7], i)) {
-			if (size > 2)
+			if (size >= 8 || (avail == 1 && size > 4))
+				wsize = 8;
+			else if (size > 2)
 				wsize = 4;
 			else
 				wsize = size;
@@ -637,6 +642,7 @@ db_md_set_watchpoint(addr, size)
 				       DBREG_DR7_WRONLY, &d);
 			addr += wsize;
 			size -= wsize;
+			avail--;
 		}
 	}
 
@@ -699,8 +705,12 @@ db_md_list_watchpoints()
 		if (DBREG_DR7_ENABLED(d.dr[7], i)) {
 			type = DBREG_DR7_ACCESS(d.dr[7], i);
 			len = DBREG_DR7_LEN(d.dr[7], i);
+			if (len == DBREG_DR7_LEN_8)
+				len = 8;
+			else
+				len++;
 			db_printf("  %-5d  %-8s  %10s  %3d  ",
-			    i, "enabled", watchtype_str(type), len + 1);
+			    i, "enabled", watchtype_str(type), len);
 			db_printsym((db_addr_t)DBREG_DRX((&d), i), DB_STGY_ANY);
 			db_printf("\n");
 		} else {
diff --git a/sys/amd64/amd64/machdep.c b/sys/amd64/amd64/machdep.c
index 6c9801029894..67469e0b282d 100644
--- a/sys/amd64/amd64/machdep.c
+++ b/sys/amd64/amd64/machdep.c
@@ -1711,7 +1711,8 @@ set_dbregs(struct thread *td, struct dbreg *dbregs)
 		for (i = 0; i < 4; i++) {
 			if (DBREG_DR7_ACCESS(dbregs->dr[7], i) == 0x02)
 				return (EINVAL);
-			if (DBREG_DR7_LEN(dbregs->dr[7], i) == 0x02)
+			if (td->td_frame->tf_cs == _ucode32sel &&
+			    DBREG_DR7_LEN(dbregs->dr[7], i) == DBREG_DR7_LEN_8)
 				return (EINVAL);
 		}
 		if ((dbregs->dr[6] & 0xffffffff00000000ul) != 0 ||
diff --git a/sys/amd64/include/reg.h b/sys/amd64/include/reg.h
index 9358f056b69d..9ca092fea104 100644
--- a/sys/amd64/include/reg.h
+++ b/sys/amd64/include/reg.h
@@ -97,6 +97,7 @@ struct dbreg {
 #define	DBREG_DR7_LEN_1		0x00	/* 1 byte length          */
 #define	DBREG_DR7_LEN_2		0x01
 #define	DBREG_DR7_LEN_4		0x03
+#define	DBREG_DR7_LEN_8		0x02
 #define	DBREG_DR7_EXEC		0x00	/* break on execute       */
 #define	DBREG_DR7_WRONLY	0x01	/* break on write         */
 #define	DBREG_DR7_RDWR		0x03	/* break on read or write */