Index: head/sys/boot/efi/include/efilib.h
===================================================================
--- head/sys/boot/efi/include/efilib.h
+++ head/sys/boot/efi/include/efilib.h
@@ -82,6 +82,7 @@
 EFI_DEVICE_PATH *efi_devpath_last_node(EFI_DEVICE_PATH *);
 EFI_DEVICE_PATH *efi_devpath_trim(EFI_DEVICE_PATH *);
 bool efi_devpath_match(EFI_DEVICE_PATH *, EFI_DEVICE_PATH *);
+int efi_devpath_is_prefix(EFI_DEVICE_PATH *, EFI_DEVICE_PATH *);
 CHAR16 *efi_devpath_name(EFI_DEVICE_PATH *);
 void efi_free_devpath_name(CHAR16 *);
 
Index: head/sys/boot/efi/libefi/devpath.c
===================================================================
--- head/sys/boot/efi/libefi/devpath.c
+++ head/sys/boot/efi/libefi/devpath.c
@@ -166,3 +166,32 @@
 	}
 	return (true);
 }
+
+int
+efi_devpath_is_prefix(EFI_DEVICE_PATH *prefix, EFI_DEVICE_PATH *path)
+{
+	int len;
+
+	if (prefix == NULL || path == NULL)
+		return (0);
+
+	while (1) {
+		if (IsDevicePathEnd(prefix))
+			break;
+
+		if (DevicePathType(prefix) != DevicePathType(path) ||
+		    DevicePathSubType(prefix) != DevicePathSubType(path))
+			return (0);
+
+		len = DevicePathNodeLength(prefix);
+		if (len != DevicePathNodeLength(path))
+			return (0);
+
+		if (memcmp(prefix, path, (size_t)len) != 0)
+			return (0);
+
+		prefix = NextDevicePathNode(prefix);
+		path = NextDevicePathNode(path);
+	}
+	return (1);
+}