Add context switch

Author: Turtel216

scheduler/mod.rs

@@ -0,0 +1,30 @@
+pub mod task;
+
+use crate::task::{Task, TaskContext, TASKS};
+use alloc::vec::Vec;
+
+/// Create a new user task (simple bare-metal “function as process”)
+pub fn spawn(entry: extern "C" fn() -> !) {
+    const STACK_SIZE: usize = 4096 * 4;
+    let mut stack: Vec<u8> = Vec::with_capacity(STACK_SIZE);
+    let stack_top = stack.as_mut_ptr() as u64 + STACK_SIZE as u64;
+
+    // Build initial context so that when we switch to it,
+    // it “returns” into the function entry().
+    let context = TaskContext {
+        r15: 0,
+        r14: 0,
+        r13: 0,
+        r12: 0,
+        rbx: 0,
+        rbp: 0,
+        rip: entry as u64,
+    };
+
+    let task = Task {
+        context,
+        stack_top,
+        page_table: x86_64::registers::control::Cr3::read(),
+    };
+    TASKS.lock().push(task);
+}

scheduler/task.rs

@@ -0,0 +1,60 @@
+use core::arch::asm;
+use x86_64::registers::control::Cr3;
+use x86_64::structures::idt::InterruptStackFrame;
+
+// Saved CPU state for a context-switchable task
+#[repr(C)]
+pub struct TaskContext {
+    // General-purpose registers (callee-saved first, per SysV ABI)
+    r15: u64,
+    r14: u64,
+    r13: u64,
+    r12: u64,
+    rbx: u64,
+    rbp: u64,
+    // Instruction pointer to resume execution
+    rip: u64,
+}
+
+// Simplest task descriptor: just context + stack + page table
+pub struct Task {
+    pub context: TaskContext,
+    pub stack_top: u64,
+    pub page_table: Cr3,
+}
+
+// The global task list and current index
+use spin::Mutex;
+static TASKS: Mutex<Vec<Task>> = Mutex::new(Vec::new());
+static mut CURRENT: usize = 0;
+
+/// Performs the low-level switch.
+/// Saves callee-saved regs + RIP into `old`, then loads from `new`.
+#[naked]
+pub unsafe extern "C" fn switch_context(old: *mut TaskContext, new: *const TaskContext) {
+    asm!(
+        // Save callee-saved registers onto the *old* context struct
+        "mov [rdi + 0x00], r15",
+        "mov [rdi + 0x08], r14",
+        "mov [rdi + 0x10], r13",
+        "mov [rdi + 0x18], r12",
+        "mov [rdi + 0x20], rbx",
+        "mov [rdi + 0x28], rbp",
+        // Store the return RIP (address after the call to `switch_context`)
+        "lea rax, [rip + 0f]",
+        "mov [rdi + 0x30], rax",
+        // Load new context
+        "mov r15, [rsi + 0x00]",
+        "mov r14, [rsi + 0x08]",
+        "mov r13, [rsi + 0x10]",
+        "mov r12, [rsi + 0x18]",
+        "mov rbx, [rsi + 0x20]",
+        "mov rbp, [rsi + 0x28]",
+        "mov rax, [rsi + 0x30]",
+        // Jump to new RIP
+        "jmp rax",
+        "0:",
+        "ret",
+        options(noreturn)
+    )
+}