test: add a way to build initramfs

Use busybox instead of a full Alpine image. This results in a much
smaller image.

In addition, we leverage `busybox devmem` command to write the magic
boot value into MMIO, removing the need for `init.c`.

Signed-off-by: Pablo Barbáchano <pablob@amazon.com>

Author: pb8o

resources/rebuild.sh

@@ -94,6 +94,45 @@ EOF
 }
 
 
+# https://wiki.gentoo.org/wiki/Custom_Initramfs#Busybox
+function build_initramfs {
+    INITRAMFS_BUILD=initramfs
+    mkdir -p $INITRAMFS_BUILD
+    pushd $INITRAMFS_BUILD
+    mkdir bin dev proc sys
+    cp /bin/busybox bin/sh
+    ln bin/sh bin/mount
+
+    # Report guest boot time back to Firecracker via MMIO
+    # See arch/src/lib.rs and the BootTimer device
+    MAGIC_BOOT_ADDRESS=0xd0000000
+    if [ $ARCH = "aarch64" ]; then
+        MAGIC_BOOT_ADDRESS=0x40000000
+    fi
+    MAGIC_BOOT_VALUE=123
+    cat > init <<EOF
+#!/bin/sh
+mount -t devtmpfs devtmpfs /dev
+mount -t proc none /proc
+devmem $MAGIC_BOOT_ADDRESS 8 $MAGIC_BOOT_VALUE
+mount -t sysfs none /sys
+exec 0</dev/console
+exec 1>/dev/console
+exec 2>/dev/console
+
+echo Boot took $(cut -d' ' -f1 /proc/uptime) seconds
+echo ">>> Welcome to fcinitrd <<<"
+
+exec /bin/sh
+EOF
+    chmod +x init
+
+    find . -print0 |cpio --null -ov --format=newc -R 0:0 > $OUTPUT_DIR/initramfs.cpio
+    popd
+    rm -rf $INITRAMFS_BUILD
+}
+
+
 function get_linux_git {
     # git clone -s -b v$KV ../../linux
     # --depth 1
@@ -175,6 +214,8 @@ fi
 # build_rootfs ubuntu-20.04 focal
 build_rootfs ubuntu-22.04 jammy
 
+build_initramfs
+
 build_linux $PWD/guest_configs/microvm-kernel-ci-$ARCH-4.14.config
 build_linux $PWD/guest_configs/microvm-kernel-ci-$ARCH-5.10.config
 

tests/conftest.py

@@ -440,13 +440,15 @@ def artifact_dir():
 
 
 @pytest.fixture
-def uvm_with_initrd(microvm_factory, guest_kernel_linux_4_14, record_property):
+def uvm_with_initrd(
+    microvm_factory, guest_kernel_linux_5_10, record_property, artifact_dir
+):
     """
     See file:../docs/initrd.md
     """
-    fs = defs.ARTIFACT_DIR / "minimal_with_initrd/initrd.img"
+    fs = artifact_dir / "initramfs.cpio"
     record_property("rootfs", fs.name)
-    uvm = microvm_factory.build(guest_kernel_linux_4_14)
+    uvm = microvm_factory.build(guest_kernel_linux_5_10)
     uvm.initrd_file = fs
     yield uvm
 
@@ -463,4 +465,3 @@ def uvm_legacy(microvm_factory, record_property, artifact_dir):
 
 # backwards compatibility
 test_microvm_with_api = uvm_plain
-test_microvm_with_initrd = uvm_with_initrd

tests/integration_tests/functional/test_initrd.py

@@ -7,11 +7,11 @@
 INITRD_FILESYSTEM = "rootfs"
 
 
-def test_microvm_initrd_with_serial(test_microvm_with_initrd):
+def test_microvm_initrd_with_serial(uvm_with_initrd):
     """
     Test that a boot using initrd successfully loads the root filesystem.
     """
-    vm = test_microvm_with_initrd
+    vm = uvm_with_initrd
     vm.jailer.daemonize = False
     vm.spawn()
     vm.memory_monitor = None
@@ -26,13 +26,6 @@ def test_microvm_initrd_with_serial(test_microvm_with_initrd):
     vm.start()
     serial = Serial(vm)
     serial.open()
-    serial.rx(token="login: ")
-    serial.tx("root")
-
-    serial.rx(token="Password: ")
-    serial.tx("root")
-
     serial.rx(token="# ")
-
-    serial.tx("findmnt /")
-    serial.rx(token=f"/      {INITRD_FILESYSTEM} {INITRD_FILESYSTEM}")
+    serial.tx("mount |grep rootfs")
+    serial.rx(token=f"rootfs on / type {INITRD_FILESYSTEM}")

tests/integration_tests/performance/test_boottime.py

@@ -100,11 +100,11 @@ def test_boottime_with_network(fast_microvm, record_property, metrics):
     ), f"boot time {boottime_us} cannot be greater than: {MAX_BOOT_TIME_US} us"
 
 
-def test_initrd_boottime(test_microvm_with_initrd, record_property, metrics):
+def test_initrd_boottime(uvm_with_initrd, record_property, metrics):
     """
     Check boot time of microVM when using an initrd.
     """
-    vm = test_microvm_with_initrd
+    vm = uvm_with_initrd
     vm.jailer.extra_args.update({"boot-timer": None})
     _configure_and_run_vm(vm, initrd=True)
     boottime_us = _get_microvm_boottime(vm)