Writing an OS in Rust (Second Edition)

Philipp Oppermann's blog

Integration Tests

In this post we complete the testing picture by implementing a basic integration test framework, which allows us to run tests on the target system. The idea is to run tests inside QEMU and report the results back to the host through the serial port.

This blog is openly developed on Github. If you have any problems or questions, please open an issue there. You can also leave comments at the bottom.

πŸ”— Overview

In the previous post we added support for unit tests. The goal of unit tests is to test small components in isolation to ensure that each of them works as intended. The tests are run on the host machine and thus shouldn't rely on architecture specific functionality.

To test the interaction of the components, both with each other and the system environment, we can write integration tests. Compared to unit tests, Γ¬ntegration tests are more complex, because they need to run in a realistic environment. What this means depends on the application type. For example, for webserver applications it often means to set up a database instance. For an operating system kernel like ours, it means that we run the tests on the target hardware without an underlying operating system.

Running on the target architecture allows us to test all hardware specific code such as the VGA buffer or the effects of page table modifications. It also allows us to verify that our kernel boots without problems and that no CPU exception occurs.

In this post we will implement a very basic test framework that runs integration tests inside instances of the QEMU virtual machine. It is not as realistic as running them on real hardware, but it is much simpler and should be sufficient as long as we only use standard hardware that is well supported in QEMU.

πŸ”— The Serial Port

The naive way of doing an integration test would be to add some assertions in the code, launch QEMU, and manually check if a panic occured or not. This is very cumbersome and not practical if we have hundreds of integration tests. So we want an automated solution that runs all tests and fails if not all of them pass.

Such an automated test framework needs to know whether a test succeeded or failed. It can't look at the screen output of QEMU, so we need a different way of retrieving the test results on the host system. A simple way to achieve this is by using the serial port, an old interface standard which is no longer found in modern computers. It is easy to program and QEMU can redirect the bytes sent over serial to the host's standard output or a file.

The chips implementing a serial interface are called UARTs. There are lots of UART models on x86, but fortunately the only differences between them are some advanced features we don't need. The common UARTs today are all compatible to the 16550 UART, so we will use that model for our testing framework.

πŸ”— Port I/O

There are two different approaches for communicating between the CPU and peripheral hardware on x86, memory-mapped I/O and port-mapped I/O. We already used memory-mapped I/O for accessing the VGA text buffer through the memory address 0xb8000. This address is not mapped to RAM, but to some memory on the GPU.

In contrast, port-mapped I/O uses a separate I/O bus for communication. Each connected peripheral has one or more port numbers. To communicate with such an I/O port there are special CPU instructions called in and out, which take a port number and a data byte (there are also variations of these commands that allow sending an u16 or u32).

The UART uses port-mapped I/O. Fortunately there are already several crates that provide abstractions for I/O ports and even UARTs, so we don't need to invoke the in and out assembly instructions manually.

πŸ”— Implementation

We will use the uart_16550 crate to initialize the UART and send data over the serial port. To add it as a dependency, we update our Cargo.toml and main.rs:

# in Cargo.toml

[dependencies]
uart_16550 = "0.1.0"
// in src/main.rs

extern crate uart_16550;

The uart_16550 crate contains a SerialPort struct that represents the UART registers, but we still need to construct an instance of it ourselves. For that we create a new serial module with the following content:

// in src/main.rs

mod serial;
// in src/serial.rs

use uart_16550::SerialPort;
use spin::Mutex;

lazy_static! {
    pub static ref SERIAL1: Mutex<SerialPort> = {
        let mut serial_port = SerialPort::new(0x3F8);
        serial_port.init();
        Mutex::new(serial_port)
    };
}

Like with the VGA text buffer, we use lazy_static and a spinlock to create a static. However, this time we use lazy_static to ensure that the init method is called before first use. We're using the port address 0x3F8, which is the standard port number for the first serial interface.

To make the serial port easily usable, we add serial_print! and serial_println! macros:

pub fn print(args: ::core::fmt::Arguments) {
    use core::fmt::Write;
    SERIAL1.lock().write_fmt(args).expect("Printing to serial failed");
}

/// Prints to the host through the serial interface.
macro_rules! serial_print {
    ($($arg:tt)*) => {
        $crate::serial::print(format_args!($($arg)*));
    };
}

/// Prints to the host through the serial interface, appending a newline.
macro_rules! serial_println {
    () => (serial_print!("\n"));
    ($fmt:expr) => (serial_print!(concat!($fmt, "\n")));
    ($fmt:expr, $($arg:tt)*) => (serial_print!(concat!($fmt, "\n"), $($arg)*));
}

The SerialPort type already implements the fmt::Write trait, so we don't need to provide an implementation.

Now we can print to the serial interface in our main.rs:

// in src/main.rs

#[macro_use]
mod serial;

#[cfg(not(test))]
#[no_mangle]
pub extern "C" fn _start() -> ! {
    println!("Hello World{}", "!"); // prints to vga buffer
    serial_println!("Hello Host{}", "!");

    loop {}
}

Note that we need to add the #[macro_use] attribute to the mod serial declaration, because otherwise the serial_println macro is not imported.

πŸ”— QEMU Arguments

To see the serial output in QEMU, we can use the -serial argument to redirect the output to stdout:

> qemu-system-x86_64 \
    -drive format=raw,file=target/x86_64-blog_os/debug/bootimage-blog_os.bin \
    -serial mon:stdio
warning: TCG doesn't support requested feature: CPUID.01H:ECX.vmx [bit 5]
Hello Host!

If you chose a different name than blog_os, you need to update the paths of course. Note that you can no longer exit QEMU through Ctrl+c. As an alternative you can use Ctrl+a and then x.

As an alternative to this long command, we can pass the argument to bootimage run, with an additional -- to separate the build arguments (passed to cargo) from the run arguments (passed to QEMU).

bootimage run -- -serial mon:stdio

Instead of standard output, QEMU supports many more target devices. For redirecting the output to a file, the argument is:

-serial file:output-file.txt

πŸ”— Shutting Down QEMU

Right now we have an endless loop at the end of our _start function and need to close QEMU manually. This does not work for automated tests. We could try to kill QEMU automatically from the host, for example after some special output was sent over serial, but this would be a bit hacky and difficult to get right. The cleaner solution would be to implement a way to shutdown our OS. Unfortunatly this is relatively complex, because it requires implementing support for either the APM or ACPI power management standard.

Luckily, there is an escape hatch: QEMU supports a special isa-debug-exit device, which provides an easy way to exit QEMU from the guest system. To enable it, we add the following argument to our QEMU command:

-device isa-debug-exit,iobase=0xf4,iosize=0x04

The iobase specifies on which port address the device should live (0xf4 is a generally unused port on the x86's IO bus) and the iosize specifies the port size (0x04 means four bytes). Now the guest can write a value to the 0xf4 port and QEMU will exit with exit status (passed_value << 1) | 1.

To write to the I/O port, we use the x86_64 crate:

# in Cargo.toml

[dependencies]
x86_64 = "0.2.8"
// in src/main.rs

extern crate x86_64;

pub unsafe fn exit_qemu() {
    use x86_64::instructions::port::Port;

    let mut port = Port::<u32>::new(0xf4);
    port.write(0);
}

We mark the function as unsafe because it relies on the fact that a special QEMU device is attached to the I/O port with address 0xf4. For the port type we choose u32 because the iosize is 4 bytes. As value we write a zero, which causes QEMU to exit with exit status (0 << 1) | 1 = 1.

Note that we could also use the exit status instead of the serial interface for sending the test results, for example 1 for success and 2 for failure. However, this wouldn't allow us to send panic messages like the serial interface does and would also prevent us from replacing exit_qemu with a proper shutdown someday. Therefore we continue to use the serial interface and just always write a 0 to the port.

We can now test the QEMU shutdown by calling exit_qemu from our _start function:

#[cfg(not(test))]
#[no_mangle]
pub extern "C" fn _start() -> ! {
    println!("Hello World{}", "!"); // prints to vga buffer
    serial_println!("Hello Host{}", "!");

    unsafe { exit_qemu(); }

    loop {}
}

You should see that QEMU immediately closes after booting when executing:

bootimage run -- -serial mon:stdio -device isa-debug-exit,iobase=0xf4,iosize=0x04

πŸ”— Hiding QEMU

We are now able to launch a QEMU instance that writes its output to the serial port and automatically exits itself when it's done. So we no longer need the VGA buffer output or the graphical representation that still pops up. We can disable it by passing the -display none parameter to QEMU. The full command looks like this:

qemu-system-x86_64 \
    -drive format=raw,file=target/x86_64-blog_os/debug/bootimage-blog_os.bin \
    -serial mon:stdio \
    -device isa-debug-exit,iobase=0xf4,iosize=0x04 \
    -display none

Or, with bootimage run:

bootimage run -- \
    -serial mon:stdio \
    -device isa-debug-exit,iobase=0xf4,iosize=0x04 \
    -display none

Now QEMU runs completely in the background and no window is opened anymore. This is not only less annoying, but also allows our test framework to run in environments without a graphical user interface, such as Travis CI.

πŸ”— Test Organization

Right now we're doing the serial output and the QEMU exit from the _start function in our main.rs and can no longer run our kernel in a normal way. We could try to fix this by adding an integration-test cargo feature and using conditional compilation:

# in Cargo.toml

[features]
integration-test = []
// in src/main.rs

#[cfg(not(feature = "integration-test"))] // new
#[cfg(not(test))]
#[no_mangle]
pub extern "C" fn _start() -> ! {
    println!("Hello World{}", "!"); // prints to vga buffer

    // normal execution

    loop {}
}

#[cfg(feature = "integration-test")] // new
#[cfg(not(test))]
#[no_mangle]
pub extern "C" fn _start() -> ! {
    serial_println!("Hello Host{}", "!");

    run_test_1();
    run_test_2();
    // run more tests

    unsafe { exit_qemu(); }

    loop {}
}

However, this approach has a big problem: All tests run in the same kernel instance, which means that they can influence each other. For example, if run_test_1 misconfigures the system by loading an invalid page table, it can cause run_test_2 to fail. This isn't something that we want because it makes it very difficult to find the actual cause of an error.

Instead, we want our test instances to be as independent as possible. If a test wants to destroy most of the system configuration to ensure that some property still holds in catastrophic situations, it should be able to do so without needing to restore a correct system state afterwards. This means that we need to launch a separate QEMU instance for each test.

With the above conditional compilation we only have two modes: Run the kernel normally or execute all integration tests. To run each test in isolation we would need a separate cargo feature for each test with that approach, which would result in very complex conditional compilation bounds and confusing code.

A better solution is to create an additional executable for each test.

πŸ”— Additional Test Executables

Cargo allows to add additional executables to a project by putting them inside src/bin. We can use that feature to create a separate executable for each integration test. For example, a test-something executable could be added like this:

// src/bin/test-something.rs

#![feature(panic_implementation)]
#![no_std]
#![cfg_attr(not(test), no_main)]
#![cfg_attr(test, allow(dead_code, unused_macros, unused_imports))]

use core::panic::PanicInfo;

#[cfg(not(test))]
#[no_mangle]
pub extern "C" fn _start() -> ! {
    // run tests
    loop {}
}

#[cfg(not(test))]
#[panic_implementation]
#[no_mangle]
pub fn panic(_info: &PanicInfo) -> ! {
    loop {}
}

By providing a new implementation for _start we can create a minimal test case that only tests one specific thing and is independent of the rest. For example, if we don't print anything to the VGA buffer, the test still succeeds even if the vga_buffer module is broken.

We can now run this executable in QEMU by passing a --bin argument to bootimage:

bootimage run --bin test-something

It should build the test-something.rs executable instead of main.rs and launch an empty QEMU window (since we don't print anything). So this approach allows us to create completely independent executables without cargo features or conditional compilation, and without cluttering our main.rs.

However, there is a problem: This is a completely separate executable, which means that we can't access any functions from our main.rs, including serial_println and exit_qemu. Duplicating the code would work, but we would also need to copy everything we want to test. This would mean that we no longer test the original function but only a possibly outdated copy.

Fortunately there is a way to share most of the code between our main.rs and the testing binaries: We move most of the code from our main.rs to a library that we can include from all executables.

πŸ”— Split Off A Library

Cargo supports hybrid projects that are both a library and a binary. We only need to create a src/lib.rs file and split the contents of our main.rs in the following way:

// src/lib.rs

#![no_std] // don't link the Rust standard library

extern crate bootloader_precompiled;
extern crate spin;
extern crate volatile;
#[macro_use]
extern crate lazy_static;
extern crate uart_16550;
extern crate x86_64;

#[cfg(test)]
extern crate array_init;
#[cfg(test)]
extern crate std;

// NEW: We need to add `pub` here to make them accessible from the outside
pub mod vga_buffer;
pub mod serial;

pub unsafe fn exit_qemu() {
    use x86_64::instructions::port::Port;

    let mut port = Port::<u32>::new(0xf4);
    port.write(0);
}
// src/main.rs

#![feature(panic_implementation)] // required for defining the panic handler
#![no_std] // don't link the Rust standard library
#![cfg_attr(not(test), no_main)] // disable all Rust-level entry points
#![cfg_attr(test, allow(dead_code, unused_macros, unused_imports))]

// NEW: Add the library as dependency (same crate name as executable)
#[macro_use]
extern crate blog_os;

use core::panic::PanicInfo;

/// This function is the entry point, since the linker looks for a function
/// named `_start` by default.
#[cfg(not(test))]
#[no_mangle] // don't mangle the name of this function
pub extern "C" fn _start() -> ! {
    println!("Hello World{}", "!");

    loop {}
}

/// This function is called on panic.
#[cfg(not(test))]
#[panic_implementation]
#[no_mangle]
pub fn panic(info: &PanicInfo) -> ! {
    println!("{}", info);
    loop {}
}

So we move everything except _start and panic to lib.rs, make the vga_buffer and serial modules public, and add an extern crate definition to our main.rs.

This doesn't compile yet, because Rust's macros are not exported over crate boundaries by default. To export our printing macros, we need to add the #[macro_export] attribute to them:

// in src/vga_buffer.rs

#[macro_export]
macro_rules! print {…}

#[macro_export]
macro_rules! println {…}

// in src/serial.rs

#[macro_export]
macro_rules! serial_print {…}

#[macro_export]
macro_rules! serial_println {…}

Now everything should work exactly as before, including bootimage run and cargo test.

πŸ”— Test Basic Boot

We are finally able to create our first integration test executable. We start simple and only test that the basic boot sequence works and the _start function is called:

// in src/bin/test-basic-boot.rs

#![feature(panic_implementation)] // required for defining the panic handler
#![no_std] // don't link the Rust standard library
#![cfg_attr(not(test), no_main)] // disable all Rust-level entry points
#![cfg_attr(test, allow(dead_code, unused_macros, unused_imports))]

// add the library as dependency (same crate name as executable)
#[macro_use]
extern crate blog_os;

use core::panic::PanicInfo;
use blog_os::exit_qemu;

/// This function is the entry point, since the linker looks for a function
/// named `_start` by default.
#[cfg(not(test))]
#[no_mangle] // don't mangle the name of this function
pub extern "C" fn _start() -> ! {
    serial_println!("ok");

    unsafe { exit_qemu(); }
    loop {}
}


/// This function is called on panic.
#[cfg(not(test))]
#[panic_implementation]
#[no_mangle]
pub fn panic(info: &PanicInfo) -> ! {
    serial_println!("failed");

    serial_println!("{}", info);

    unsafe { exit_qemu(); }
    loop {}
}

We don't do something special here, we just print ok if _start is called and failed with the panic message when a panic occurs. Let's try it:

> bootimage run --bin test-basic-boot -- \
    -serial mon:stdio -display none \
    -device isa-debug-exit,iobase=0xf4,iosize=0x04
Building kernel
   Compiling blog_os v0.2.0 (file:///…/blog_os)
    Finished dev [unoptimized + debuginfo] target(s) in 0.19s
    Updating registry `https://github.com/rust-lang/crates.io-index`
Creating disk image at target/x86_64-blog_os/debug/bootimage-test-basic-boot.bin
warning: TCG doesn't support requested feature: CPUID.01H:ECX.vmx [bit 5]
ok

We got our ok, so it worked! Try inserting a panic!() before the ok printing, you should see output like this:

failed
panicked at 'explicit panic', src/bin/test-basic-boot.rs:19:5

πŸ”— Test Panic

To test that our panic handler is really invoked on a panic, we create a test-panic test:

// in src/bin/test-panic.rs

#![feature(panic_implementation)]
#![no_std]
#![cfg_attr(not(test), no_main)]
#![cfg_attr(test, allow(dead_code, unused_macros, unused_imports))]

#[macro_use]
extern crate blog_os;

use core::panic::PanicInfo;
use blog_os::exit_qemu;

#[cfg(not(test))]
#[no_mangle]
pub extern "C" fn _start() -> ! {
    panic!();
}

#[cfg(not(test))]
#[panic_implementation]
#[no_mangle]
pub fn panic(_info: &PanicInfo) -> ! {
    serial_println!("ok");

    unsafe { exit_qemu(); }
    loop {}
}

This executable is almost identical to test-basic-boot, the only difference is that we print ok from our panic handler and invoke an explicit panic() in our _start function.

πŸ”— A Test Runner

The final step is to create a test runner, a program that executes all integration tests and checks their results. The basic steps that it should do are:

Such a test runner is useful to many projects, so we decided to add one to the bootimage tool.

πŸ”— Bootimage Test

The test runner of the bootimage tool can be invoked via bootimage test. It uses the following conventions:

The test-basic-boot and test-panic tests we created above begin with test- and follow the ok/failed conventions, so they should work with bootimage test:

> bootimage test
test-panic
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
Ok

test-basic-boot
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
Ok

test-something
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
Timed Out

The following tests failed:
    test-something: TimedOut

We see that our test-panic and test-basic-boot succeeded and that the test-something test timed out after one minute. We no longer need test-something, so we delete it (if you haven't done already). Now bootimage test should execute successfully.

πŸ”— Summary

In this post we learned about the serial port and port-mapped I/O and saw how to configure QEMU to print serial output to the command line. We also learned a trick how to exit QEMU without needing to implement a proper shutdown.

We then split our crate into a library and binary part in order to create additional executables for integration tests. We added two example tests for testing that the _start function is correctly called and that a panic invokes our panic handler. Finally, we presented bootimage test as a basic test runner for our integration tests.

We now have a working integration test framework and can finally start to implement functionality in our kernel. We will continue to use the test framework over the next posts to test new components we add.

πŸ”— What's next?

In the next post, we will explore CPU exceptions. These exceptions are thrown by the CPU when something illegal happens, such as a division by zero or an access to an unmapped memory page (a so-called β€œpage fault”). Being able to catch and examine these exceptions is very important for debugging future errors. Exception handling is also very similar to the handling of hardware interrupts, which is required for keyboard support.



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