Add qemu serial output to stdio

Signed-off-by: Noah Knegt <git@noahknegt.com>

Cargo.lock

@@ -17,6 +17,12 @@ version = "1.0.98"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e16d2d3311acee920a9eb8d33b8cbc1787ce4a264e85f964c2404b969bdcd487"
 
+[[package]]
+name = "autocfg"
+version = "1.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c08606f8c3cbf4ce6ec8e28fb0014a2c086708fe954eaa885384a6165172e7e8"
+
 [[package]]
 name = "bincode"
 version = "1.3.3"
@@ -26,6 +32,12 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "bit_field"
+version = "0.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dc827186963e592360843fb5ba4b973e145841266c1357f7180c43526f2e5b61"
+
 [[package]]
 name = "bitflags"
 version = "1.3.2"
@@ -99,6 +111,21 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9555578bc9e57714c812a1f84e4fc5b4d21fcb063490c624de019f7464c91268"
 
+[[package]]
+name = "conquer-once"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d008a441c0f269f36ca13712528069a86a3e60dffee1d98b976eb3b0b2160b4"
+dependencies = [
+ "conquer-util",
+]
+
+[[package]]
+name = "conquer-util"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e763eef8846b13b380f37dfecda401770b0ca4e56e95170237bd7c25c7db3582"
+
 [[package]]
 name = "crc"
 version = "3.3.0"
@@ -192,6 +219,12 @@ name = "kernel"
 version = "0.1.0"
 dependencies = [
  "bootloader_api",
+ "conquer-once",
+ "log",
+ "noto-sans-mono-bitmap",
+ "spinning_top",
+ "uart_16550",
+ "x86_64",
 ]
 
 [[package]]
@@ -212,6 +245,16 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "955be5d0ca0465caf127165acb47964f911e2bc26073e865deb8be7189302faf"
 
+[[package]]
+name = "lock_api"
+version = "0.4.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "96936507f153605bddfcda068dd804796c84324ed2510809e5b2a624c81da765"
+dependencies = [
+ "autocfg",
+ "scopeguard",
+]
+
 [[package]]
 name = "log"
 version = "0.4.27"
@@ -237,6 +280,12 @@ version = "2.7.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "32a282da65faaf38286cf3be983213fcf1d2e2a58700e808f83f4ea9a4804bc0"
 
+[[package]]
+name = "noto-sans-mono-bitmap"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1064d564ae026ae123bf5d607318b42a5b31d70a3c48e6ea7ee44cce4cdb095e"
+
 [[package]]
 name = "once_cell"
 version = "1.21.3"
@@ -279,6 +328,15 @@ version = "0.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dc33ff2d4973d518d823d61aa239014831e521c75da58e3df4840d3f47749d09"
 
+[[package]]
+name = "raw-cpuid"
+version = "10.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6c297679cb867470fa8c9f67dbba74a78d78e3e98d7cf2b08d6d71540f797332"
+dependencies = [
+ "bitflags 1.3.2",
+]
+
 [[package]]
 name = "rustix"
 version = "1.0.8"
@@ -304,6 +362,12 @@ version = "1.0.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "28d3b2b1366ec20994f1fd18c3c594f05c5dd4bc44d8bb0c1c632c8d6829481f"
 
+[[package]]
+name = "scopeguard"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
+
 [[package]]
 name = "serde"
 version = "1.0.219"
@@ -345,6 +409,15 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "spinning_top"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d96d2d1d716fb500937168cc09353ffdc7a012be8475ac7308e1bdf0e3923300"
+dependencies = [
+ "lock_api",
+]
+
 [[package]]
 name = "syn"
 version = "2.0.104"
@@ -395,6 +468,17 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "uart_16550"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e492212ac378a5e00da953718dafb1340d9fbaf4f27d6f3c5cab03d931d1c049"
+dependencies = [
+ "bitflags 2.9.1",
+ "rustversion",
+ "x86",
+]
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.18"
@@ -412,6 +496,12 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "volatile"
+version = "0.4.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "442887c63f2c839b346c192d047a7c87e73d0689c9157b00b53dcc27dd5ea793"
+
 [[package]]
 name = "wasi"
 version = "0.14.2+wasi-0.2.4"
@@ -642,3 +732,26 @@ checksum = "05f360fc0b24296329c78fda852a1e9ae82de9cf7b27dae4b7f62f118f77b9ed"
 dependencies = [
  "tap",
 ]
+
+[[package]]
+name = "x86"
+version = "0.52.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2781db97787217ad2a2845c396a5efe286f87467a5810836db6d74926e94a385"
+dependencies = [
+ "bit_field",
+ "bitflags 1.3.2",
+ "raw-cpuid",
+]
+
+[[package]]
+name = "x86_64"
+version = "0.15.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0f042214de98141e9c8706e8192b73f56494087cc55ebec28ce10f26c5c364ae"
+dependencies = [
+ "bit_field",
+ "bitflags 2.9.1",
+ "rustversion",
+ "volatile",
+]

LICENSE

@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2025 noah.knegt
+Copyright (c) 2025 Noah Knegt <personal@noahknegt.com>
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of this software and
 associated documentation files (the "Software"), to deal in the Software without restriction, including

kernel/Cargo.toml

@@ -10,3 +10,19 @@ publish.workspace = true
 
 [dependencies]
 bootloader_api = { version = "0.11.10" }
+conquer-once = { version = "0.4.0", default-features = false }
+log = "0.4.17"
+spinning_top = "0.3.0"
+uart_16550 = "0.3.2"
+x86_64 = "0.15.2"
+
+[dependencies.noto-sans-mono-bitmap]
+version = "0.3.1"
+default-features = false
+features = [
+    "regular",
+    "size_16",
+    "unicode-basic-latin",
+    # required for the fallback char '<27>'
+    "unicode-specials",
+]

kernel/src/logger/framebuffer.rs

@@ -0,0 +1,154 @@
+use bootloader_api::info::{FrameBufferInfo, PixelFormat};
+use core::{fmt, ptr};
+use font_constants::BACKUP_CHAR;
+use noto_sans_mono_bitmap::{
+    get_raster, get_raster_width, FontWeight, RasterHeight, RasterizedChar,
+};
+
+/// Additional vertical space between lines
+const LINE_SPACING: usize = 2;
+/// Additional horizontal space between characters.
+const LETTER_SPACING: usize = 0;
+
+/// Padding from the border. Prevent that font is too close to border.
+const BORDER_PADDING: usize = 1;
+
+/// Constants for the usage of the [`noto_sans_mono_bitmap`] crate.
+mod font_constants {
+    use super::*;
+
+    /// Height of each char raster. The font size is ~0.84% of this. Thus, this is the line height that
+    /// enables multiple characters to be side-by-side and appear optically in one line in a natural way.
+    pub const CHAR_RASTER_HEIGHT: RasterHeight = RasterHeight::Size16;
+
+    /// The width of each single symbol of the mono space font.
+    pub const CHAR_RASTER_WIDTH: usize = get_raster_width(FontWeight::Regular, CHAR_RASTER_HEIGHT);
+
+    /// Backup character if a desired symbol is not available by the font.
+    /// The '<27>' character requires the feature "unicode-specials".
+    pub const BACKUP_CHAR: char = '<27>';
+
+    pub const FONT_WEIGHT: FontWeight = FontWeight::Regular;
+}
+
+/// Returns the raster of the given char or the raster of [`font_constants::BACKUP_CHAR`].
+fn get_char_raster(c: char) -> RasterizedChar {
+    fn get(c: char) -> Option<RasterizedChar> {
+        get_raster(
+            c,
+            font_constants::FONT_WEIGHT,
+            font_constants::CHAR_RASTER_HEIGHT,
+        )
+    }
+    get(c).unwrap_or_else(|| get(BACKUP_CHAR).expect("Should get raster of backup char."))
+}
+
+/// Allows logging text to a pixel-based framebuffer.
+pub struct FrameBufferWriter {
+    framebuffer: &'static mut [u8],
+    info: FrameBufferInfo,
+    x_pos: usize,
+    y_pos: usize,
+}
+
+impl FrameBufferWriter {
+    /// Creates a new logger that uses the given framebuffer.
+    pub fn new(framebuffer: &'static mut [u8], info: FrameBufferInfo) -> Self {
+        let mut logger = Self {
+            framebuffer,
+            info,
+            x_pos: 0,
+            y_pos: 0,
+        };
+        logger.clear();
+        logger
+    }
+
+    fn newline(&mut self) {
+        self.y_pos += font_constants::CHAR_RASTER_HEIGHT.val() + LINE_SPACING;
+        self.carriage_return()
+    }
+
+    fn carriage_return(&mut self) {
+        self.x_pos = BORDER_PADDING;
+    }
+
+    /// Erases all text on the screen. Resets `self.x_pos` and `self.y_pos`.
+    pub fn clear(&mut self) {
+        self.x_pos = BORDER_PADDING;
+        self.y_pos = BORDER_PADDING;
+        self.framebuffer.fill(0);
+    }
+
+    fn width(&self) -> usize {
+        self.info.width
+    }
+
+    fn height(&self) -> usize {
+        self.info.height
+    }
+
+    /// Writes a single char to the framebuffer. Takes care of special control characters, such as
+    /// newlines and carriage returns.
+    fn write_char(&mut self, c: char) {
+        match c {
+            '\n' => self.newline(),
+            '\r' => self.carriage_return(),
+            c => {
+                let new_xpos = self.x_pos + font_constants::CHAR_RASTER_WIDTH;
+                if new_xpos >= self.width() {
+                    self.newline();
+                }
+                let new_ypos =
+                    self.y_pos + font_constants::CHAR_RASTER_HEIGHT.val() + BORDER_PADDING;
+                if new_ypos >= self.height() {
+                    self.clear();
+                }
+                self.write_rendered_char(get_char_raster(c));
+            }
+        }
+    }
+
+    /// Prints a rendered char into the framebuffer.
+    /// Updates `self.x_pos`.
+    fn write_rendered_char(&mut self, rendered_char: RasterizedChar) {
+        for (y, row) in rendered_char.raster().iter().enumerate() {
+            for (x, byte) in row.iter().enumerate() {
+                self.write_pixel(self.x_pos + x, self.y_pos + y, *byte);
+            }
+        }
+        self.x_pos += rendered_char.width() + LETTER_SPACING;
+    }
+
+    fn write_pixel(&mut self, x: usize, y: usize, intensity: u8) {
+        let pixel_offset = y * self.info.stride + x;
+        let color = match self.info.pixel_format {
+            PixelFormat::Rgb => [intensity, intensity, intensity / 2, 0],
+            PixelFormat::Bgr => [intensity / 2, intensity, intensity, 0],
+            PixelFormat::U8 => [if intensity > 200 { 0xf } else { 0 }, 0, 0, 0],
+            other => {
+                // set a supported (but invalid) pixel format before panicking to avoid a double
+                // panic; it might not be readable though
+                self.info.pixel_format = PixelFormat::Rgb;
+                panic!("pixel format {:?} not supported in logger", other)
+            }
+        };
+        let bytes_per_pixel = self.info.bytes_per_pixel;
+        let byte_offset = pixel_offset * bytes_per_pixel;
+        self.framebuffer[byte_offset..(byte_offset + bytes_per_pixel)]
+            .copy_from_slice(&color[..bytes_per_pixel]);
+        let _ = unsafe { ptr::read_volatile(&self.framebuffer[byte_offset]) };
+    }
+}
+
+unsafe impl Send for FrameBufferWriter {}
+unsafe impl Sync for FrameBufferWriter {}
+
+impl fmt::Write for FrameBufferWriter {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        for c in s.chars() {
+            self.write_char(c);
+        }
+        Ok(())
+    }
+}

kernel/src/logger/logger.rs

@@ -0,0 +1,95 @@
+use bootloader_api::info::FrameBufferInfo;
+use conquer_once::spin::OnceCell;
+use core::fmt::Write;
+use spinning_top::Spinlock;
+
+use super::*;
+
+/// The global logger instance used for the `log` crate.
+static LOGGER: OnceCell<LockedLogger> = OnceCell::uninit();
+
+/// A logger instance protected by a spinlock.
+struct LockedLogger {
+    framebuffer: Option<Spinlock<framebuffer::FrameBufferWriter>>,
+    serial: Option<Spinlock<serial::SerialPort>>
+}
+
+impl LockedLogger {
+    /// Create a new instance that logs to the given framebuffer.
+    pub fn new(
+        framebuffer: &'static mut [u8],
+        info: FrameBufferInfo,
+    ) -> Self {
+        let framebuffer = Some(Spinlock::new(framebuffer::FrameBufferWriter::new(framebuffer, info)));
+        let serial = Some(Spinlock::new(unsafe { serial::SerialPort::init() }));
+
+        LockedLogger {
+            framebuffer,
+            serial
+        }
+    }
+
+    /// Force-unlocks the logger to prevent a deadlock.
+    ///
+    /// ## Safety
+    /// This method is not memory safe and should be only used when absolutely necessary.
+    pub unsafe fn force_unlock(&self) {
+        if let Some(framebuffer) = &self.framebuffer {
+            unsafe { framebuffer.force_unlock() };
+        }
+
+        if let Some(serial) = &self.serial {
+            unsafe { serial.force_unlock() };
+        }
+    }
+}
+
+impl log::Log for LockedLogger {
+    fn enabled(&self, _metadata: &log::Metadata) -> bool {
+        true
+    }
+
+    fn log(&self, record: &log::Record) {
+        if let Some(framebuffer) = &self.framebuffer {
+            let mut framebuffer = framebuffer.lock();
+            writeln!(framebuffer, "{:5}: {}", record.level(), record.args()).unwrap();
+        }
+
+        if let Some(serial) = &self.serial {
+            let mut serial = serial.lock();
+            writeln!(serial, "{:5}: {}", record.level(), record.args()).unwrap();
+        }
+    }
+
+    fn flush(&self) {}
+}
+
+/// Initialize a text-based logger using the given pixel-based framebuffer as output.
+pub fn init_logger(
+    framebuffer: &'static mut [u8],
+    info: FrameBufferInfo,
+) {
+    let logger = LOGGER.get_or_init(move || {
+        LockedLogger::new(
+            framebuffer,
+            info,
+        )
+    });
+    log::set_logger(logger).expect("logger already set");
+    #[cfg(debug_assertions)]
+    log::set_max_level(log::LevelFilter::Debug);
+
+    #[cfg(not(debug_assertions))]
+    log::set_max_level(log::LevelFilter::Info);
+
+    log::debug!("Framebuffer info: {:?}", info);
+}
+
+pub fn force_unlock() {
+    unsafe {
+        LOGGER
+            .get()
+            .map(|l| l.force_unlock())
+    };
+}
+

kernel/src/logger/mod.rs

@@ -0,0 +1,5 @@
+pub mod logger;
+mod framebuffer;
+mod serial;
+
+pub use logger::*;

kernel/src/logger/serial.rs

@@ -0,0 +1,28 @@
+use core::fmt;
+
+pub struct SerialPort {
+    port: uart_16550::SerialPort,
+}
+
+impl SerialPort {
+    /// # Safety
+    ///
+    /// unsafe because this function must only be called once
+    pub unsafe fn init() -> Self {
+        let mut port = unsafe { uart_16550::SerialPort::new(0x3F8) };
+        port.init();
+        Self { port }
+    }
+}
+
+impl fmt::Write for SerialPort {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        for char in s.bytes() {
+            match char {
+                b'\n' => self.port.write_str("\r\n").unwrap(),
+                byte => self.port.send(byte),
+            }
+        }
+        Ok(())
+    }
+}

kernel/src/main.rs

@@ -1,16 +1,37 @@
 #![no_std]
 #![no_main]
 
-use bootloader_api::{entry_point, BootInfo};
+use bootloader_api::{
+    config::{BootloaderConfig, Mapping},
+    entry_point, BootInfo
+};
+
+mod logger;
+
+pub static BOOTLOADER_CONFIG: BootloaderConfig = {
+    let mut config = BootloaderConfig::new_default();
+    config.mappings.physical_memory = Some(Mapping::Dynamic);
+    config
+};
 
 entry_point!(kernel_main);
 
 fn kernel_main(boot_info: &'static mut BootInfo) -> ! {
+    let info = boot_info.framebuffer.as_ref().unwrap().info();
+    let buffer = boot_info.framebuffer.as_mut().unwrap().buffer_mut();
+
+    logger::init_logger(buffer, info);
+
+    log::info!("Hello World from KERNEL");
+
+    // Endless loop as the kernel must stay running
     loop {}
 }
 
 #[panic_handler]
 #[cfg(not(test))]
 fn panic(info: &core::panic::PanicInfo) -> ! {
+    logger::force_unlock();
+    log::error!("{}", info);
     loop {}
 }

rust-toolchain.toml

@@ -1,5 +1,7 @@
 [toolchain]
-channel = "nightly"
+# The nightly version that was build on 24-07-2025 is the last known working version,
+# the c-int-width definition for rust targets has changed from a string to a u16
+channel = "nightly-2025-07-24"
 targets = [
     "x86_64-unknown-none"
 ]

src/main.rs

@@ -11,10 +11,15 @@ fn main() {
     let mut cmd = std::process::Command::new("qemu-system-x86_64");
     if uefi {
         cmd.arg("-bios").arg(ovmf_prebuilt::ovmf_pure_efi());
-        cmd.arg("-drive").arg(format!("format=raw,file={uefi_path}"));
+        cmd.arg("-drive")
+            .arg(format!("format=raw,file={uefi_path}"));
     } else {
-        cmd.arg("-drive").arg(format!("format=raw,file={bios_path}"));
+        cmd.arg("-drive")
+            .arg(format!("format=raw,file={bios_path}"));
     }
+
+    cmd.arg("-serial").arg("stdio");
+
     let mut child = cmd.spawn().unwrap();
     child.wait().unwrap();
 }