diff --git a/tools/build/Makefile.feature b/tools/build/Makefile.feature
index 805631543186..52600e4d33af 100644
--- a/tools/build/Makefile.feature
+++ b/tools/build/Makefile.feature
@@ -28,6 +28,41 @@ endef
 #   the rule that uses them - an example for that is the 'bionic'
 #   feature check. ]
 #
+# These + the ones in FEATURE_TESTS_EXTRA are included in
+# tools/build/feature/test-all.c and we try to build it all together
+# then setting all those features to '1' meaning they are all enabled.
+#
+# There are things like fortify-source that will be set to 1 because test-all
+# is built with the flags needed to test if its enabled, resulting in
+#
+#   $ rm -rf /tmp/b ; mkdir /tmp/b ; make -C tools/perf O=/tmp/b feature-dump
+#   $ grep fortify-source /tmp/b/FEATURE-DUMP
+#   feature-fortify-source=1
+#   $
+#
+#   All the others should have lines in tools/build/feature/test-all.c like:
+#
+#    #define main main_test_disassembler_init_styled
+#    # include "test-disassembler-init-styled.c"
+#    #undef main
+#
+#    #define main main_test_libzstd
+#    # include "test-libzstd.c"
+#    #undef main
+#
+#    int main(int argc, char *argv[])
+#    {
+#      main_test_disassembler_four_args();
+#      main_test_libzstd();
+#      return 0;
+#    }
+#
+#    If the sample above works, then we end up with these lines in the FEATURE-DUMP
+#    file:
+#
+#    feature-disassembler-four-args=1
+#    feature-libzstd=1
+#
 FEATURE_TESTS_BASIC :=                  \
         backtrace                       \
         libdw                           \