Attribute Macro main 

#[main]

A macro attribute to mark the kernel entry point.

Bootstrap Context

After initializing itself on the BSP, OSTD transfers control to this kernel entrypoint function marked with #[ostd::main]. Symmetrically, each AP also has a kernel entrypoint function, which is specified with ostd::boot::smp::register_ap_entry.

Both BSP and AP kernel entrypoints execute code in the bootstrap context, which is intended to provide an opportunity for an OSTD-based kernel to perform all sorts of necessary initialization steps before spawning kernel/user tasks to execute code in the task context.

The bootstrap context has the following limitations:

1. No current task: Unlike the task context, the bootstrap context is not associated with a task. So calling Task::current in the bootstrap context always gets a None.
2. No turning back: The kernel code execution eventually transitions from the bootstrap context to the task context. This transition is triggered by doing return or by calling Task::yield_now in a kernel entrypoint function. Either way, there is no turning back once leaving the bootstrap context.

Custom Task Scheduler

One key initialization step is to set up a custom task scheduler. To do so, the bootstrap code should initialize the task scheduler and then enable it by injecting it into OSTD (see ostd::task::scheduler::inject_scheduler). If no scheduler is injected until a scheduler-related operation (e.g., Task::yield_now) is triggered, then the default FIFO task scheduler will be used by OSTD.

Examples

The high-level workflow of a typical entrypoint function is demonstrated below:

use ostd::boot::smp;
use ostd::task::{Task, TaskOptions};

#[ostd::main]
fn bsp_entrypoint() {
    // Call `ostd::task::scheduler::inject_scheduler` here
    // to specify a custom scheduler.

    // Perform initialization steps on BSP here.

    smp::register_ap_entry(ap_entrypoint);

    TaskOptions::new(idle_task_fn).spawn();
    // Returning from here transitions to the task context.
}

fn ap_entrypoint() {
    // Perform initialization steps on AP here.

    TaskOptions::new(idle_task_fn).spawn().unwrap();
    // Returning from here transitions to the task context.
}

fn idle_task_fn() {
    loop {
        Task::yield_now();
    }
}