diff --git a/.gitmodules b/.gitmodules
index ee2019a..d274169 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,6 +1,7 @@
 [submodule "prebuilts"]
 	path = prebuilts
 	url = sso://hafnium/hafnium/prebuilts
+	shallow = true
 [submodule "driver/linux"]
 	path = driver/linux
 	url = sso://hafnium/hafnium/driver/linux
diff --git a/Makefile b/Makefile
index e1227e7..8ad1562 100644
--- a/Makefile
+++ b/Makefile
@@ -1,12 +1,13 @@
 OUT ?= out
 
-# Set path to prebuilts
+# Set path to prebuilts used in the build.
 UNNAME_S := $(shell uname -s | tr '[:upper:]' '[:lower:]')
-PREBUILTS := prebuilts/$(UNNAME_S)-x64
+PREBUILTS := $(PWD)/prebuilts/$(UNNAME_S)-x64
 GN ?= $(PREBUILTS)/gn/gn
 NINJA ?= $(PREBUILTS)/ninja/ninja
+export PATH := $(PREBUILTS)/clang/bin:$(PATH)
 
-# Configure the build arguments
+# Configure the build arguments.
 GCC ?= false
 ARCH ?= aarch64
 PLATFORM ?= qemu
@@ -19,7 +20,7 @@
 	@$(GN) --export-compile-commands gen $(OUT)
 
 # Configure the build by loading the configuration arguments for the
-# architecture and platform
+# architecture and platform.
 $(OUT)/args.gn: build/arch/$(ARCH)/$(PLATFORM).args
 	@echo Copying config for $(ARCH) on $(PLATFORM)
 	@mkdir -p $(OUT)
@@ -36,7 +37,7 @@
 clobber:
 	rm -rf $(OUT)
 
-# see .clang-format
+# see .clang-format.
 .PHONY: format
 format:
 	@echo "Formatting..."
@@ -45,7 +46,7 @@
 	@find test/ -name *.c -o -name *.h | xargs clang-format -style file -i
 	@find . -name *.gn -o -name *.gni -exec $(GN) format {} \;
 
-# see .clang-tidy
+# see .clang-tidy.
 .PHONY: tidy
 tidy: $(OUT)/build.ninja
 	@$(NINJA) -C $(OUT)
diff --git a/build/BUILD.gn b/build/BUILD.gn
index d383359..8ef2258 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn
@@ -46,12 +46,6 @@
       "-ffunction-sections",
     ]
 
-    if (!use_gcc) {
-      cflags += [
-        "-Wno-extended-offsetof", # have clang give us some slack
-      ]
-    }
-
     defines += [
       # TODO: move these build args for the platforms
       "MAX_CPUS=8",
diff --git a/build/toolchain/arch/BUILD.gn b/build/toolchain/arch/BUILD.gn
index 6523339..b04ed4c 100644
--- a/build/toolchain/arch/BUILD.gn
+++ b/build/toolchain/arch/BUILD.gn
@@ -77,7 +77,7 @@
   cc_toolchain(target_name) {
     cc = "clang"
     cflags = "-target ${invoker.target} -fcolor-diagnostics"
-    ld = "${invoker.arch_tool_prefix}ld"
+    ld = "ld.lld"
 
     toolchain_args = {
       arch_tool_prefix = invoker.arch_tool_prefix
diff --git a/kokoro/ubuntu/build.sh b/kokoro/ubuntu/build.sh
index 95da482..4596757 100755
--- a/kokoro/ubuntu/build.sh
+++ b/kokoro/ubuntu/build.sh
@@ -12,23 +12,36 @@
 	# Server
 	cd git/hafnium
 	make out/args.gn
-	echo "arch_cc_version = \"3.9\"" >> out/args.gn
-	echo "host_cc_version = \"3.9\"" >> out/args.gn
 else
 	# Local
 	echo "Testing kokoro build locally..."
 	make out/args.gn
 fi
 
-# TODO: Kokoro is missing ninja, gcc-4.9 or above and qemu
-# Check the build works
-make
-
-# # Check to code looks healthy, failing if any changes were made
-# make format
-# make tidy
 #
-# if [[ `git status --porcelain` ]]
-# then
-# 	exit 1
-# fi
+# Step 1: make sure it builds.
+#
+
+# TODO: add a gcc-4.9 or above prebuilt to check the gcc build too?
+# Check the build works.
+make
+#TODO: static analysis with make check
+
+#
+# Step 2: make sure it works.
+#
+
+./kokoro/ubuntu/test.sh
+
+#
+# Step 3: make sure the code looks good.
+#
+
+make format
+make tidy
+
+if [[ `git status --porcelain` ]]
+then
+	echo "Run `make format` and `make tidy` locally to fix this."
+	exit 1
+fi
diff --git a/kokoro/ubuntu/test.sh b/kokoro/ubuntu/test.sh
new file mode 100755
index 0000000..c313d6d
--- /dev/null
+++ b/kokoro/ubuntu/test.sh
@@ -0,0 +1,18 @@
+#!/bin/bash
+
+# Note: this assumes that the images have all been built and the current working
+# directory is the root of the repo.
+
+# Fail on any error.
+set -e
+# Display commands being run.
+set -x
+
+TIMEOUT="timeout --foreground"
+QEMU="./prebuilts/linux-x64/qemu/qemu-system-aarch64 -M virt -cpu cortex-a57 -m 8M -machine virtualization=true -nographic -nodefaults -serial stdio"
+HAFNIUM="out/clang_aarch64/hafnium.bin"
+INITRD="out/clang_aarch64/initrd"
+
+# Run the QEMU tests with a timeout so they can't loop forever.
+$TIMEOUT 5s $QEMU -kernel $HAFNIUM -initrd $INITRD/primary_only_test.img
+$TIMEOUT 5s $QEMU -kernel $HAFNIUM -initrd $INITRD/primary_with_secondary_test.img
diff --git a/prebuilts b/prebuilts
index 7ea7831..5e1ddfa 160000
--- a/prebuilts
+++ b/prebuilts
@@ -1 +1 @@
-Subproject commit 7ea7831e75ae22fa0a1467c302e656819132a6fd
+Subproject commit 5e1ddfae19f7052e9e27b1bf2c2d8ea3c9bcf5b0