diff --git a/usr.sbin/bhyve/basl.h b/usr.sbin/bhyve/basl.h
--- a/usr.sbin/bhyve/basl.h
+++ b/usr.sbin/bhyve/basl.h
@@ -44,5 +44,6 @@
     uint8_t bit_width, uint8_t bit_offset, uint8_t access_width,
     uint64_t address);
 int basl_table_append_int(struct basl_table *table, uint64_t val, uint8_t size);
+int basl_table_append_length(struct basl_table *table, uint8_t size);
 int basl_table_create(struct basl_table **table, struct vmctx *ctx,
     const uint8_t name[QEMU_FWCFG_MAX_NAME], uint32_t alignment, uint32_t off);
diff --git a/usr.sbin/bhyve/basl.c b/usr.sbin/bhyve/basl.c
--- a/usr.sbin/bhyve/basl.c
+++ b/usr.sbin/bhyve/basl.c
@@ -36,6 +36,15 @@
 STAILQ_HEAD(basl_table_list, basl_table) basl_tables = STAILQ_HEAD_INITIALIZER(
     basl_tables);
 
+struct basl_table_length {
+	STAILQ_ENTRY(basl_table_length) chain;
+	struct basl_table *table;
+	uint32_t off;
+	uint8_t size;
+};
+STAILQ_HEAD(basl_table_length_list,
+    basl_table_length) basl_lengths = STAILQ_HEAD_INITIALIZER(basl_lengths);
+
 struct qemu_loader *basl_loader;
 
 static int
@@ -121,6 +130,21 @@
 	return (0);
 }
 
+static int
+basl_finish_set_length()
+{
+	struct basl_table_length *length;
+	STAILQ_FOREACH (length, &basl_lengths, chain) {
+		const struct basl_table *const table = length->table;
+
+		uint32_t len_le = htole32(table->len);
+
+		memcpy(table->data + length->off, &len_le, length->size);
+	}
+
+	return (0);
+}
+
 int
 basl_finish()
 {
@@ -129,6 +153,7 @@
 		return (EINVAL);
 	}
 
+	BASL_EXEC(basl_finish_set_length());
 	BASL_EXEC(basl_finish_alloc());
 
 	return (0);
@@ -140,6 +165,26 @@
 	return (0);
 }
 
+static int
+basl_table_add_length(struct basl_table *const table, const uint32_t off,
+    const uint8_t size)
+{
+	struct basl_table_length *const length = calloc(1,
+	    sizeof(struct basl_table_length));
+	if (length == NULL) {
+		warnx("%s: failed to allocate length", __func__);
+		return (ENOMEM);
+	}
+
+	length->table = table;
+	length->off = off;
+	length->size = size;
+
+	STAILQ_INSERT_TAIL(&basl_lengths, length, chain);
+
+	return (0);
+}
+
 int
 basl_table_append_bytes(struct basl_table *const table, const void *const bytes,
     const uint32_t len)
@@ -196,6 +241,19 @@
 	return (basl_table_append_bytes(table, &val_le, size));
 }
 
+int
+basl_table_append_length(struct basl_table *const table, const uint8_t size)
+{
+	if (table == NULL || size > sizeof(table->len)) {
+		return (EINVAL);
+	}
+
+	BASL_EXEC(basl_table_add_length(table, table->len, size));
+	BASL_EXEC(basl_table_append_int(table, 0, size));
+
+	return (0);
+}
+
 int
 basl_table_create(struct basl_table **const table, struct vmctx *ctx,
     const uint8_t name[QEMU_FWCFG_MAX_NAME], const uint32_t alignment,