diff --git a/HACKING.md b/HACKING.md
index 460427b8afb..dd04b25d9f8 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -15,3 +15,6 @@ There are several gradle flags one can use for Konan build.
 
         ./gradlew -Pfilter=overflowLong.kt run_external
 
+* **-Pperfix** allows one to choose external test directories to run. Only tests from directories with given prefix will be executed.
+
+        ./gradlew -Pfilter=external_codegen_box_cast run_external
\ No newline at end of file
diff --git a/README.md b/README.md
index 6924416bb1d..85a9665fd66 100644
--- a/README.md
+++ b/README.md
@@ -22,4 +22,10 @@ For some tests, use:
 
 To run blackbox compiler tests from JVM Kotlin use (takes time):
 
-    ./gradlew run_external
\ No newline at end of file
+    ./gradlew run_external
+
+To update the blackbox compiler tests set TeamCity build number in `gradle.properties`:
+
+    testDataVersion=<build number>:id
+
+and run `./gradlew update_external_tests`
\ No newline at end of file
diff --git a/backend.native/tests/build.gradle b/backend.native/tests/build.gradle
index 35de3a12a1d..511a88d517d 100644
--- a/backend.native/tests/build.gradle
+++ b/backend.native/tests/build.gradle
@@ -44,6 +44,7 @@ allprojects {
     }
 }
 
+// TODO: use lastSuccessful teamcity build.
 task update_external_tests() {
     doLast {
         // Unzip.