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

Index: util/ectool/ectool.c
--- util/ectool/ectool.c.orig
+++ util/ectool/ectool.c
@@ -130,7 +130,7 @@ int main(int argc, char *argv[])
 				"fallback to default.\n");
 
 	if (iopl(3)) {
-		printf("You need to be root.\n");
+		printf("You need to be root and machdep.allowaperture must be non-zero, see amd64_iopl(2).\n");
 		exit(1);
 	}
 	if (write_addr >= 0 && write_data >= 0) {
