Add an Overview of the Framework page

docs/src/framework/a-100-line-kernel.md

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+# Example: Writing a Kernel in About 100 Lines of Safe Rust
+
+To give you a sense of
+how Asterinas Framework enables writing kernels in safe Rust,
+we will show a new kernel in about 100 lines of safe Rust.
+
+Our new kernel will be able to run the following Hello World program.
+
+```s
+global _start
+
+section .text
+
+_start:
+    mov rax, 1        ; syswrite
+    mov rdi, 1        ; fd
+    mov rsi, msg      ; "Hello, world!\n",
+    mov rdx, msglen   ; sizeof("Hello, world!\n")
+    syscall
+
+    mov rax, 60       ; sys_exit
+    mov rdi, 0        ; exit_code
+    syscall
+
+section .rodata
+    msg: db "Hello, world!", 10
+```
+
+The user program above requires our kernel to support three main features:
+1. Loading a program as a process image in user space;
+3. Handling the write system call;
+4. Handling the exit system call.
+
+A sample implementation of the kernel in safe Rust is given below.
+Comments are added
+to highlight how the APIs of Asterinas Framework enable safe kernel development.
+
+```rust
+#![no_std]
+
+extern crate alloc;
+
+use alloc::boxed::Box;
+use alloc::collections::VecDeque;
+use alloc::sync::Arc;
+use alloc::vec;
+
+use aster_frame::cpu::UserContext;
+use aster_frame::prelude::*;
+use aster_frame::task::{Task, TaskOptions};
+use aster_frame::user::{UserEvent, UserMode, UserSpace};
+use aster_frame::vm::{Vaddr, VmAllocOptions, VmIo, VmMapOptions, VmPerm, VmSpace};
+
+/// The kernel's boot and initialization process is managed by Asterinas Framework.
+/// After the process is done, the kernel's execution environment
+/// (e.g., stack, heap, tasks) will be ready for use and the entry function
+/// labeled as `#[aster_main]` will be called.
+#[aster_main]
+pub fn main() {
+    let program_binary = include_bytes!("../hello_world");
+    let user_space = create_user_space(program_binary);
+    let user_task = create_user_task(user_space);
+    user_task.run();
+}
+
+fn create_user_space(program: &[u8]) -> UserSpace {
+    let user_pages = {
+        let nframes = content.len().align_up(PAGE_SIZE) / PAGE_SIZE;
+        let vm_frames = VmAllocOptions::new(nframes).alloc().unwrap();
+        // Phyiscal memory pages can be only accessed
+        // via the VmFrame abstraction.
+        frames.write_bytes(0, program).unwrap()
+    };
+    let user_address_space = {
+        const MAP_ADDR: Vaddr = 0x0040_0000; // The map addr for statically-linked executable
+
+        // The page table of the user space can be
+        // created and manipulated safely through
+        // the VmSpace abstraction.
+        let vm_space = VmSpace::new();
+        let mut options = VmMapOptions::new();
+        options.addr(Some(MAP_ADDR)).perm(VmPerm::RWX);
+        vm_space.map(user_pages, &options).unwrap();
+        vm_space
+    };
+    let user_cpu_state = {
+        const ENTRY_POINT: Vaddr = 0x0040_1000; // The entry point for statically-linked executable
+
+        // The user-space CPU states can be initialized
+        // to arbitrary values via the UserContext
+        // abstraction.
+        let mut user_cpu_state = UserContext::default();
+        user_cpu_state.set_rip(ENTRY_POINT);
+    };
+    UserSpace::new(user_address_space, user_cpu_state)
+}
+
+fn create_user_task(user_space: Arc<UserSpace>) -> Arc<Task> {
+    fn user_task() {
+        let current = Task::current();
+        // Switching between user-kernel space is
+        // performed via the UserMode abstraction.
+        let mut user_mode = {
+            let user_space = current.user_space().unwrap();
+            UserMode::new(user_space)
+        };
+
+        loop {
+            // The execute method returns when system
+            // calls or CPU exceptions occur.
+            let user_event = user_mode.execute();
+            // The CPU registers of the user space
+            // can be accessed and manipulated via
+            // the `UserContext` abstraction.
+            let user_context = user_mode.context_mut();
+            if UserEvent::Syscall == user_event {
+                handle_syscall(user_context, user_space);
+            }
+        }
+    }
+
+    // Kernel tasks are managed by the Framework,
+    // while scheduling algorithms for them can be
+    // determined by the users of the Framework.
+    TaskOptions::new(user_task)
+        .user_space(Some(user_space))
+        .data(0)
+        .build()
+        .unwrap()
+}
+
+fn handle_syscall(user_context: &mut UserContext, user_space: &UserSpace) {
+    const SYS_WRITE: usize = 1;
+    const SYS_EXIT: usize = 60;
+
+    match user_context.rax() {
+        SYS_WRITE => {
+            // Access the user-space CPU registers safely.
+            let (fd, buf_addr, buf_len) =
+                (user_context.rdi(), user_context.rsi(), user_context.rdx());
+            let buf = {
+                let mut buf = vec![0u8; buf_len];
+                // Copy data from the user space without
+                // unsafe pointer dereferencing.
+                user_space
+                    .vm_space()
+                    .read_bytes(buf_addr, &mut buf)
+                    .unwrap();
+            };
+            // Use the console for output safely.
+            println!("{}", str::from_utf8(&buf).unwrap());
+            // Manipulate the user-space CPU registers safely.
+            user_context.set_rax(buf_len);
+        }
+        SYS_EXIT => Task::current().exit(),
+        _ => unimplemented!(),
+    }
+}
+```
+