diff --git a/make/ModuleTools.gmk b/make/ModuleTools.gmk
index c48a1b6ec3d..020e12d6e4b 100644
--- a/make/ModuleTools.gmk
+++ b/make/ModuleTools.gmk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2013, 2017, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2013, 2020, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # This code is free software; you can redistribute it and/or modify it
@@ -44,7 +44,7 @@ TOOL_GENGRAPHS := $(BUILD_JAVA) -esa -ea -cp $(TOOLS_CLASSES_DIR) \
 TOOL_MODULESUMMARY := $(BUILD_JAVA) -esa -ea -cp $(TOOLS_CLASSES_DIR) \
     build.tools.jigsaw.ModuleSummary
 
-TOOL_ADD_PACKAGES_ATTRIBUTE := $(BUILD_JAVA) $(JAVA_FLAGS_SMALL) \
+TOOL_ADD_PACKAGES_ATTRIBUTE := $(BUILD_JAVA) $(JAVA_FLAGS_SMALL_BUILDJDK) \
     -cp $(TOOLS_CLASSES_DIR) \
     --add-exports java.base/jdk.internal.module=ALL-UNNAMED \
     build.tools.jigsaw.AddPackagesAttribute
diff --git a/make/RunTestsPrebuiltSpec.gmk b/make/RunTestsPrebuiltSpec.gmk
index dc533a01266..94126548910 100644
--- a/make/RunTestsPrebuiltSpec.gmk
+++ b/make/RunTestsPrebuiltSpec.gmk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2017, 2019, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2017, 2020, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # This code is free software; you can redistribute it and/or modify it
@@ -63,8 +63,9 @@ TEST_JOBS ?= 0
 
 # Use hard-coded values for java flags (one size, fits all!)
 JAVA_FLAGS := -Duser.language=en -Duser.country=US
-JAVA_FLAGS_BIG:= -Xms64M -Xmx1600M -XX:ThreadStackSize=1536
-JAVA_FLAGS_SMALL:= -XX:+UseSerialGC -Xms32M -Xmx512M -XX:TieredStopAtLevel=1
+JAVA_FLAGS_BIG := -Xms64M -Xmx1600M -XX:ThreadStackSize=1536
+JAVA_FLAGS_SMALL := -XX:+UseSerialGC -Xms32M -Xmx512M -XX:TieredStopAtLevel=1
+BUILDJDK_JAVA_FLAGS_SMALL := -Xms32M -Xmx512M -XX:TieredStopAtLevel=1
 BUILD_JAVA_FLAGS := $(JAVA_FLAGS_BIG)
 
 ################################################################################
diff --git a/make/autoconf/boot-jdk.m4 b/make/autoconf/boot-jdk.m4
index 874adab97b8..3383097d8c0 100644
--- a/make/autoconf/boot-jdk.m4
+++ b/make/autoconf/boot-jdk.m4
@@ -496,6 +496,11 @@ AC_DEFUN_ONCE([BOOTJDK_SETUP_BOOT_JDK_ARGUMENTS],
   JAVA_FLAGS_SMALL=$boot_jdk_jvmargs_small
   AC_SUBST(JAVA_FLAGS_SMALL)
 
+  # Don't presuppose SerialGC is present in the buildjdk. Also, we cannot test
+  # the buildjdk, but on the other hand we know what it will support.
+  BUILDJDK_JAVA_FLAGS_SMALL="-Xms32M -Xmx512M -XX:TieredStopAtLevel=1"
+  AC_SUBST(BUILDJDK_JAVA_FLAGS_SMALL)
+
   JAVA_TOOL_FLAGS_SMALL=""
   for f in $JAVA_FLAGS_SMALL; do
     JAVA_TOOL_FLAGS_SMALL="$JAVA_TOOL_FLAGS_SMALL -J$f"
diff --git a/make/autoconf/spec.gmk.in b/make/autoconf/spec.gmk.in
index 60d660b32ec..1380170c352 100644
--- a/make/autoconf/spec.gmk.in
+++ b/make/autoconf/spec.gmk.in
@@ -620,6 +620,7 @@ STRIPFLAGS:=@STRIPFLAGS@
 JAVA_FLAGS:=@JAVA_FLAGS@
 JAVA_FLAGS_BIG:=@JAVA_FLAGS_BIG@
 JAVA_FLAGS_SMALL:=@JAVA_FLAGS_SMALL@
+BUILDJDK_JAVA_FLAGS_SMALL:=@BUILDJDK_JAVA_FLAGS_SMALL@
 JAVA_FLAGS_JAVAC:=@JAVA_FLAGS_JAVAC@
 JAVA_TOOL_FLAGS_SMALL:=@JAVA_TOOL_FLAGS_SMALL@
 SJAVAC_SERVER_JAVA_FLAGS:=@SJAVAC_SERVER_JAVA_FLAGS@
diff --git a/make/hotspot/lib/JvmFeatures.gmk b/make/hotspot/lib/JvmFeatures.gmk
index 5f5e8c02342..22772493f23 100644
--- a/make/hotspot/lib/JvmFeatures.gmk
+++ b/make/hotspot/lib/JvmFeatures.gmk
@@ -55,6 +55,10 @@ ifeq ($(call check-jvm-feature, zero), true)
   endif
 endif
 
+ifeq ($(JVM_VARIANT), custom)
+  JVM_CFLAGS_FEATURES += -DVMTYPE=\"Custom\"
+endif
+
 ifeq ($(call check-jvm-feature, minimal), true)
   JVM_CFLAGS_FEATURES += -DMINIMAL_JVM -DVMTYPE=\"Minimal\"
   ifeq ($(call isTargetOs, linux), true)
@@ -151,8 +155,6 @@ endif
 ifneq ($(call check-jvm-feature, serialgc), true)
   JVM_CFLAGS_FEATURES += -DINCLUDE_SERIALGC=0
   JVM_EXCLUDE_PATTERNS += gc/serial
-  # If serial is disabled, we cannot use serial as OldGC in parallel
-  JVM_EXCLUDE_FILES += psMarkSweep.cpp psMarkSweepDecorator.cpp
 endif
 
 ifneq ($(call check-jvm-feature, epsilongc), true)