global vga buf

boot1/src/main.rs

@@ -1,12 +1,11 @@
 #![no_std]
 #![no_main]
 
+mod spin;
 mod vga;
 
 use core::{arch::asm, panic::PanicInfo, fmt::Write};
 
-use vga::VgaBuf;
-
 #[repr(C)]
 #[derive(Clone, Debug)]
 struct BiosIntr {
@@ -32,9 +31,9 @@ fn panic(_info: &PanicInfo) -> ! {
 
 #[no_mangle]
 pub extern "C" fn _start() -> ! {
-  let mut vga_buf = unsafe { VgaBuf::new(0xb8000 as *mut u16) };
+  vga::vga_init();
 
-  writeln!(&mut vga_buf, "hello from rust").ok();
+  vga_println!("hello from rust");
 
   let mut args = BiosIntr {
     eax: 0xe820,
@@ -52,7 +51,7 @@ pub extern "C" fn _start() -> ! {
     _bios_call(0x15, &raw mut args);
   }
 
-  writeln!(&mut vga_buf, "eax = {:x}", args.eax).ok();
+  vga_println!("eax = {:x}", args.eax);
 
   hlt()
 }

boot1/src/spin.rs

@@ -0,0 +1,101 @@
+use core::{cell::UnsafeCell, hint, ops::{Deref, DerefMut}, sync::atomic::{AtomicBool, Ordering}};
+
+pub struct Spinlock<T> {
+  data: UnsafeCell<T>,
+  locked: AtomicBool,
+}
+
+impl<T> Spinlock<T> {
+  pub const fn new(data: T) -> Self {
+    Self {
+      data: UnsafeCell::new(data),
+      locked: AtomicBool::new(false),
+    }
+  }
+
+  pub fn lock(&self) -> SpinlockGuard<T> {
+    // If we observe `locked` was `false`, then:
+    // - Use acquire ordering, so nothing inside the critical section gets reordered before we
+    //   observed the `false` value.
+    // - Store `true`, so nothing else can enter the critical section until we exit it. 
+    // Otherwise, spin until we observe a `false` value.
+    while self.locked
+      .compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed)
+      .is_err()
+    {
+      hint::spin_loop();
+    }
+
+    SpinlockGuard { lock: self }
+  }
+
+  /// # Safety
+  /// There must be no "active" `SpinlockGuards` for this lock, i.e. a `SpinlockGuard` which can be
+  /// used to obtain a reference to the spinlock-protected data.
+  unsafe fn unlock(&self) {
+    // Unset `locked` with release ordering so that nothing inside the critical section gets
+    // reordered to after we stored `false`.
+    self.locked.store(false, Ordering::Release);
+  }
+
+  /// # Safety
+  /// No mutable references to the spinlock-protected data may exist.
+  unsafe fn get<'s, 'a>(&'s self) -> &'a T
+  where
+    's: 'a,
+  {
+    unsafe { &*self.data.get() }
+  }
+
+  /// # Safety
+  /// No references to the spinlock-protected data may exist.
+  unsafe fn get_mut<'s, 'a>(&'s self) -> &'a mut T
+  where
+    's: 'a,
+  {
+    unsafe { &mut *self.data.get() }
+  }
+}
+
+unsafe impl<T> Sync for Spinlock<T>
+where
+  T: Send,
+{}
+
+pub struct SpinlockGuard<'a, T> {
+  lock: &'a Spinlock<T>,
+}
+
+impl<'a, T> Deref for SpinlockGuard<'a, T> {
+  type Target = T;
+
+  #[inline]
+  fn deref(&self) -> &Self::Target {
+    // SAFETY:
+    // For the entire lifetime of the `SpinlockGuard`, `locked` remains `true`, so we have
+    // exclusive access to `data`, so no mutable references to `data` can exist.
+    unsafe { self.lock.get() }
+  }
+}
+
+impl<'a, T> DerefMut for SpinlockGuard<'a, T> {
+  #[inline]
+  fn deref_mut(&mut self) -> &mut Self::Target {
+    // SAFETY:
+    // For the entire lifetime of the `SpinlockGuard`, `locked` remains `true`, so we have
+    // exclusive access to `data`, so no other references to `data` can exist.
+    unsafe { self.lock.get_mut() }
+  }
+}
+
+impl<'a, T> Drop for SpinlockGuard<'a, T> {
+  fn drop(&mut self) {
+    // SAFETY:
+    // Only one `SpinlockGuard` can exist at a time for a particular lock, since we set `locked`
+    // to true before creating a guard and refuse to create any new ones until it is `false` again.
+    // Therefore, we are the only `SpinlockGuard` for the lock. Since this is the destructor, and
+    // we don't access the spinlock-protected data here, there are therefore no "active"
+    // `SpinlockGuard`s remaining for the lock.
+    unsafe { self.lock.unlock() }
+  }
+}

boot1/src/vga.rs

@@ -1,10 +1,47 @@
 use core::{fmt, ptr};
 
+use crate::spin::Spinlock;
+
 const VGA_WIDTH: usize = 80;
 const VGA_HEIGHT: usize = 25;
 
 const COLOUR_MASK: u16 = 0x0a00;
 
+pub static VGA: Spinlock<Option<VgaBuf>> = Spinlock::new(None);
+
+#[macro_export]
+macro_rules! vga_print {
+  ($($args:tt)*) => ({
+    crate::vga::with_vga_buf(|vga_buf| {
+      ::core::write!(vga_buf, $($args)*).ok();
+    });
+  })
+}
+
+#[macro_export]
+macro_rules! vga_println {
+  ($($args:tt)*) => ({
+    crate::vga::with_vga_buf(|vga_buf| {
+      ::core::writeln!(vga_buf, $($args)*).ok();
+    });
+  })
+}
+
+pub fn vga_init() {
+  let vga_buf = unsafe { VgaBuf::new(0xb8000 as *mut u16) };
+  let mut guard = VGA.lock();
+  *guard = Some(vga_buf);
+}
+
+pub fn with_vga_buf<F>(f: F)
+where
+  F: FnOnce(&mut VgaBuf),
+{
+  let mut guard = crate::vga::VGA.lock();
+  let vga = guard.as_mut().expect("vga not initialised");
+  f(vga);
+}
+
 pub struct VgaBuf {
   buf: *mut u16,
   col: usize,
@@ -87,3 +124,5 @@ impl fmt::Write for VgaBuf {
     Ok(())
   }
 }
+
+unsafe impl Send for VgaBuf {}