diff --git a/tools/libxl/libxl_arm_acpi.c b/tools/libxl/libxl_arm_acpi.c
index db113dbf85..d50ffa39c6 100644
--- a/tools/libxl/libxl_arm_acpi.c
+++ b/tools/libxl/libxl_arm_acpi.c
@@ -37,7 +37,7 @@ typedef int64_t s64;
 #define BITS_PER_LONG 32
 #endif
 #endif
-#define ACPI_MACHINE_WIDTH __BITS_PER_LONG
+#define ACPI_MACHINE_WIDTH BITS_PER_LONG
 #define COMPILER_DEPENDENT_INT64 int64_t
 #define COMPILER_DEPENDENT_UINT64 uint64_t