amd64_iopl(2) requires non-zero machdep.allowaperture sysctl(8).

Index: util/nvramtool/accessors/cmos-hw-unix.c
--- util/nvramtool/accessors/cmos-hw-unix.c.orig
+++ util/nvramtool/accessors/cmos-hw-unix.c
@@ -159,8 +159,7 @@ static void cmos_set_iopl(int level)
 	}
 #else
 	if (iopl(level)) {
-		fprintf(stderr, "%s: iopl() system call failed.  "
-			"You must be root to do this.\n", prog_name);
+		fprintf(stderr, "%s: You need to be root and machdep.allowaperture must be non-zero, see amd64_iopl(2).\n", prog_name);
 		exit(1);
 	}
 #endif
