--- old/make/conf/jib-profiles.js	2018-10-11 14:59:32.100899313 -0700
+++ new/make/conf/jib-profiles.js	2018-10-11 14:59:31.696885180 -0700
@@ -755,16 +755,15 @@
         "run-test-prebuilt": {
             target_os: input.build_os,
             target_cpu: input.build_cpu,
-            src: "src.conf",
             dependencies: [ "jtreg", "gnumake", "boot_jdk", "jib", testedProfile + ".jdk",
-                testedProfile + ".test", "src.full"
+                testedProfile + ".test"
             ],
-            work_dir: input.get("src.full", "install_path") + "/test",
+            src: "src.conf",
+            make_args: [ "run-test-prebuilt", "LOG_CMDLINES=true" ],
             environment: {
-                "JT_JAVA": common.boot_jdk_home,
-                "PRODUCT_HOME": input.get(testedProfile + ".jdk", "home_path"),
-                "TEST_IMAGE_DIR": input.get(testedProfile + ".test", "home_path"),
-                "TEST_OUTPUT_DIR": input.src_top_dir
+                "BOOT_JDK": common.boot_jdk_home,
+                "JDK_IMAGE_DIR": input.get(testedProfile + ".jdk", "home_path"),
+                "TEST_IMAGE_DIR": input.get(testedProfile + ".test", "home_path")
             },
             labels: "test"
         }
@@ -809,6 +808,27 @@
             windowsRunTestPrebuiltExtra);
     }
 
+    // The profile run-test-prebuilt defines src.conf as the src bundle. When
+    // running in Mach 5, this reduces the time it takes to populate the
+    // considerably. But with just src.conf, we cannot actually run any tests,
+    // so if running from a workspace with just src.conf in it, we need to also
+    // get src.full as a dependency, and define the work_dir (where make gets
+    // run) to be in the src.full install path. By running in the install path,
+    // the same cached installation of the full src can be reused for multiple
+    // test tasks. Care must however be taken not to polute that work dir by
+    // setting the appropriate make variables to control output directories.
+    //
+    // Use the existance of the top level README as indication of if this is
+    // the full source or just src.conf.
+    if (!new java.io.File(__DIR__, "../../README").exists()) {
+        var runTestPrebuiltSrcFullExtra = {
+            dependencies: "src.full",
+            work_dir: input.get("src.full", "install_path"),
+        }
+        profiles["run-test-prebuilt"] = concatObjects(profiles["run-test-prebuilt"],
+            runTestPrebuiltSrcFullExtra);
+    }
+
     // Generate the missing platform attributes
     profiles = generatePlatformAttributes(profiles);
     profiles = generateDefaultMakeTargetsConfigureArg(common, profiles);
@@ -835,7 +855,7 @@
                     : "gcc7.3.0-Fedora27+1.0"),
         linux_arm: (input.profile != null && input.profile.indexOf("hflt") >= 0
                     ? "gcc-linaro-arm-linux-gnueabihf-raspbian-2012.09-20120921_linux+1.0"
-                    : (input.profile.indexOf("arm32") >= 0
+                    : (input.profile != null && input.profile.indexOf("arm32") >= 0
                        ? "gcc7.3.0-Fedora27+1.0"
                        : "arm-linaro-4.7+1.0"
                        )