feat(ipc): implement synchronous Rendezvous IPC and multi-thread cooperative scheduler

include/abi/syscalls.h

@@ -45,6 +45,30 @@ static inline sysret_t sys_ipc_call_raw(cap_t dest_cap, uint64_t msg_code,
     return (sysret_t)rax;
 }
 
+static inline sysret_t sys_ipc_reply_recv_raw(cap_t listen_ep, uint64_t reply_to_tid,
+                                              uint64_t reply_val0, uint64_t reply_val1,
+                                              uint64_t *sender_tid, uint64_t *opcode,
+                                              uint64_t *arg0, uint64_t *arg1) {
+    register uint64_t rax __asm__("rax") = SYS_IPC_REPLY_RECV;
+    register uint64_t rdi __asm__("rdi") = listen_ep;
+    register uint64_t rsi __asm__("rsi") = reply_to_tid;
+    register uint64_t rdx __asm__("rdx") = reply_val0;
+    register uint64_t r8  __asm__("r8")  = reply_val1;
+
+    __asm__ volatile(
+        "syscall"
+        : "+r"(rax), "+r"(rdi), "+r"(rsi), "+r"(rdx), "+r"(r8)
+        :
+        : "rcx", "r11", "memory"
+    );
+
+    if (sender_tid) *sender_tid = rdi;
+    if (opcode) *opcode = rsi;
+    if (arg0) *arg0 = rdx;
+    if (arg1) *arg1 = r8;
+    return (sysret_t)rax;
+}
+
 static inline sysret_t sys_yield(void) {
     register uint64_t rax __asm__("rax") = SYS_THREAD_YIELD;
     __asm__ volatile("syscall" : "+r"(rax) : : "rcx", "r11", "memory");

include/abi/types.h

@@ -30,9 +30,11 @@ typedef uint32_t tid_t;        /* Thread ID */
 
 /* Standard Special Capabilities */
 #define CAP_NULL                0
+#define CAP_KERNEL_CONTROL      1
 #define CAP_SELF_TASK           1
-#define CAP_SELF_THREAD         2
+#define CAP_ROOT_SERVER         2
-#define CAP_SELF_VMSPACE        3
+#define CAP_SELF_THREAD         3
-#define CAP_ROOT_CSPACE         4
+#define CAP_SELF_VMSPACE        4
-#define CAP_ROOT_IRQ            5
+#define CAP_ROOT_CSPACE         5
-#define CAP_ROOT_IO             6
+#define CAP_ROOT_IRQ            6
+#define CAP_ROOT_IO             7

kernel/src/arch/syscall.rs

@@ -1,5 +1,8 @@
-//! x86_64 Fast Syscall / Sysret Setup and Fast-Path Dispatch
+//! x86_64 Fast Syscall / Sysret Setup and Fast-Path & Rendezvous Dispatch
 
+use crate::ipc::endpoint::EP_MANAGER;
+use crate::ipc::fastpath::{CAP_KERNEL_CONTROL, handle_fastpath_call};
+use crate::sched::SCHEDULER;
 use crate::kprintln;
 
 const IA32_EFER: u32 = 0xC0000080;
@@ -39,20 +42,13 @@ extern "C" {
 }
 
 pub unsafe fn init() {
-    // 1. Enable System Call Extension (SCE) in IA32_EFER
     let efer = rdmsr(IA32_EFER);
     wrmsr(IA32_EFER, efer | 1);
 
-    // 2. Set STAR MSR:
-    // Bits [47:32] = Kernel CS/SS selectors (0x0008)
-    // Bits [63:48] = User CS/SS selectors base (0x0010 -> SS=0x18, CS=0x20)
     let star = (0x0010u64 << 48) | (0x0008u64 << 32);
     wrmsr(IA32_STAR, star);
 
-    // 3. Set LSTAR to our assembly entry trampoline
     wrmsr(IA32_LSTAR, syscall_entry as *const () as usize as u64);
-
-    // 4. Set FMASK to clear IF (bit 9, 0x200), DF (bit 10), TF (bit 8)
     wrmsr(IA32_FMASK, 0x00000200);
 }
 
@@ -70,8 +66,13 @@ pub struct SyscallRegisters {
 /// Dispatcher called directly from syscall_entry assembly trampoline
 #[no_mangle]
 pub extern "C" fn kernel_syscall_dispatcher(regs: &mut SyscallRegisters) {
+    let current_tid = unsafe {
+        let sched = &mut *core::ptr::addr_of_mut!(SCHEDULER);
+        sched.get_current_mut().id
+    };
+
     match regs.rax {
-        // SYS_IPC_CALL = 1
+        // SYS_IPC_CALL = 1 (Client: Send & Wait for Reply)
         1 => {
             let dest_cap = regs.rdi;
             let opcode = regs.rsi;
@@ -80,16 +81,57 @@ pub extern "C" fn kernel_syscall_dispatcher(regs: &mut SyscallRegisters) {
             let arg2 = regs.r9;
             let arg3 = regs.r10;
 
-            let (ret, out0, out1) = crate::ipc::fastpath::handle_fastpath_call(
+            if dest_cap == CAP_KERNEL_CONTROL {
-                dest_cap, opcode, arg0, arg1, arg2, arg3,
+                let (ret, out0, out1) = handle_fastpath_call(dest_cap, opcode, arg0, arg1, arg2, arg3);
-            );
+                regs.rax = ret as u64;
+                regs.rdx = out0;
+                regs.r8 = out1;
+            } else {
+                // Inter-Process Rendezvous IPC Call
+                unsafe {
+                    let ep_mgr = &mut *core::ptr::addr_of_mut!(EP_MANAGER);
+                    match ep_mgr.ipc_call(dest_cap, opcode, [arg0, arg1, arg2, arg3], current_tid) {
+                        Ok((out0, out1)) => {
+                            regs.rax = 0;
+                            regs.rdx = out0;
+                            regs.r8 = out1;
+                        }
+                        Err(code) => {
+                            regs.rax = code as u64;
+                        }
+                    }
+                }
+            }
+        }
+        // SYS_IPC_REPLY_RECV = 2 (Server: Reply to Caller & Wait for Request)
+        2 => {
+            let listen_ep = regs.rdi;
+            let reply_to_tid = regs.rsi;
+            let reply_val0 = regs.rdx;
+            let reply_val1 = regs.r8;
 
-            regs.rax = ret as u64;
+            unsafe {
-            regs.rdx = out0;
+                let ep_mgr = &mut *core::ptr::addr_of_mut!(EP_MANAGER);
-            regs.r8 = out1;
+                match ep_mgr.ipc_reply_recv(listen_ep, reply_to_tid, reply_val0, reply_val1, current_tid) {
+                    Ok((sender_tid, opcode, a0, a1)) => {
+                        regs.rax = 0;
+                        regs.rdi = sender_tid;
+                        regs.rsi = opcode;
+                        regs.rdx = a0;
+                        regs.r8 = a1;
+                    }
+                    Err(code) => {
+                        regs.rax = code as u64;
+                    }
+                }
+            }
         }
         // SYS_THREAD_YIELD = 7
         7 => {
+            unsafe {
+                let sched = &mut *core::ptr::addr_of_mut!(SCHEDULER);
+                sched.yield_current();
+            }
             regs.rax = 0;
         }
         // SYS_LOG_DEBUG = 9
@@ -99,7 +141,7 @@ pub extern "C" fn kernel_syscall_dispatcher(regs: &mut SyscallRegisters) {
             if !msg_ptr.is_null() && len > 0 && len < 4096 {
                 let slice = unsafe { core::slice::from_raw_parts(msg_ptr, len) };
                 if let Ok(s) = core::str::from_utf8(slice) {
-                    kprintln!("[USER LOG] {}", s);
+                    kprintln!("[USER TID {}] {}", current_tid, s);
                     regs.rax = 0;
                     return;
                 }

kernel/src/ipc/endpoint.rs

@@ -0,0 +1,195 @@
+//! IPC Endpoints and Synchronous Rendezvous Engine
+
+use crate::ipc::fastpath::FastPathMessage;
+use crate::sched::SCHEDULER;
+use crate::sched::thread::ThreadState;
+use crate::kprintln;
+
+pub const MAX_ENDPOINTS: usize = 32;
+pub const MAX_QUEUE_PER_EP: usize = 8;
+
+#[derive(Clone, Copy)]
+pub struct Endpoint {
+    pub id: u64,
+    pub receiver_tid: u64,
+    pub send_queue: [u64; MAX_QUEUE_PER_EP],
+    pub send_count: usize,
+}
+
+impl Endpoint {
+    pub const fn empty() -> Self {
+        Self {
+            id: 0,
+            receiver_tid: 0,
+            send_queue: [0; MAX_QUEUE_PER_EP],
+            send_count: 0,
+        }
+    }
+}
+
+pub struct EndpointManager {
+    pub endpoints: [Endpoint; MAX_ENDPOINTS],
+}
+
+pub static mut EP_MANAGER: EndpointManager = EndpointManager {
+    endpoints: [const { Endpoint::empty() }; MAX_ENDPOINTS],
+};
+
+impl EndpointManager {
+    pub fn init(&mut self) {
+        for i in 0..MAX_ENDPOINTS {
+            self.endpoints[i].id = i as u64;
+        }
+    }
+
+    pub fn get_endpoint_mut(&mut self, ep_id: u64) -> Option<&mut Endpoint> {
+        let idx = ep_id as usize;
+        if idx < MAX_ENDPOINTS {
+            Some(&mut self.endpoints[idx])
+        } else {
+            None
+        }
+    }
+
+    /// Client performs synchronous Call (Send + Wait for Reply)
+    pub unsafe fn ipc_call(
+        &mut self,
+        dest_ep: u64,
+        opcode: u64,
+        args: [u64; 4],
+        caller_tid: u64,
+    ) -> Result<(u64, u64), i64> {
+        let ep = self.get_endpoint_mut(dest_ep).ok_or(-2i64)?; // Invalid Cap/Endpoint
+        let sched = &mut *core::ptr::addr_of_mut!(SCHEDULER);
+
+        if ep.receiver_tid != 0 {
+            // Rendezvous: Server is already blocked waiting for a request!
+            let server_tid = ep.receiver_tid;
+            ep.receiver_tid = 0; // Clear receiver wait state
+
+            // Transfer message directly into server's context
+            if let Some(server) = sched.get_thread_mut(server_tid) {
+                server.caller_tid = caller_tid;
+                server.ipc_buffer = FastPathMessage {
+                    dest_cap: dest_ep,
+                    opcode,
+                    args,
+                };
+                server.state = ThreadState::Ready;
+            }
+
+            // Block caller on reply from server
+            if let Some(caller) = sched.get_thread_mut(caller_tid) {
+                caller.state = ThreadState::BlockedOnReply(server_tid);
+            }
+
+            // Direct context handoff to server
+            sched.schedule();
+
+            // When unblocked after reply, return results stored in caller's buffer
+            if let Some(caller) = sched.get_thread_mut(caller_tid) {
+                return Ok((caller.ipc_buffer.args[0], caller.ipc_buffer.args[1]));
+            }
+            Ok((0, 0))
+        } else {
+            // Receiver not waiting: Enqueue sender and block
+            if ep.send_count >= MAX_QUEUE_PER_EP {
+                return Err(-9); // SYS_ERR_BUSY
+            }
+
+            ep.send_queue[ep.send_count] = caller_tid;
+            ep.send_count += 1;
+
+            if let Some(caller) = sched.get_thread_mut(caller_tid) {
+                caller.ipc_buffer = FastPathMessage {
+                    dest_cap: dest_ep,
+                    opcode,
+                    args,
+                };
+                caller.state = ThreadState::BlockedOnSend(dest_ep);
+            }
+
+            sched.schedule();
+
+            if let Some(caller) = sched.get_thread_mut(caller_tid) {
+                return Ok((caller.ipc_buffer.args[0], caller.ipc_buffer.args[1]));
+            }
+            Ok((0, 0))
+        }
+    }
+
+    /// Server handles Reply to current caller and waits for Next Request
+    pub unsafe fn ipc_reply_recv(
+        &mut self,
+        listen_ep: u64,
+        reply_to_tid: u64,
+        reply_val0: u64,
+        reply_val1: u64,
+        server_tid: u64,
+    ) -> Result<(u64, u64, u64, u64), i64> {
+        let ep = self.get_endpoint_mut(listen_ep).ok_or(-2i64)?;
+        let sched = &mut *core::ptr::addr_of_mut!(SCHEDULER);
+
+        // 1. Deliver reply to previous caller if requested
+        if reply_to_tid != 0 {
+            if let Some(caller) = sched.get_thread_mut(reply_to_tid) {
+                caller.ipc_buffer.args[0] = reply_val0;
+                caller.ipc_buffer.args[1] = reply_val1;
+                caller.state = ThreadState::Ready;
+            }
+        }
+
+        // 2. Check if a sender is already queued
+        if ep.send_count > 0 {
+            let next_sender_tid = ep.send_queue[0];
+            // Shift queue
+            for i in 0..(ep.send_count - 1) {
+                ep.send_queue[i] = ep.send_queue[i + 1];
+            }
+            ep.send_count -= 1;
+
+            let mut ret_opcode = 0;
+            let mut ret_arg0 = 0;
+            let mut ret_arg1 = 0;
+
+            if let Some(sender) = sched.get_thread_mut(next_sender_tid) {
+                ret_opcode = sender.ipc_buffer.opcode;
+                ret_arg0 = sender.ipc_buffer.args[0];
+                ret_arg1 = sender.ipc_buffer.args[1];
+                sender.state = ThreadState::BlockedOnReply(server_tid);
+            }
+
+            if let Some(srv) = sched.get_thread_mut(server_tid) {
+                srv.caller_tid = next_sender_tid;
+            }
+
+            return Ok((next_sender_tid, ret_opcode, ret_arg0, ret_arg1));
+        }
+
+        // 3. No sender queued: block server on endpoint
+        ep.receiver_tid = server_tid;
+        if let Some(srv) = sched.get_thread_mut(server_tid) {
+            srv.state = ThreadState::BlockedOnRecv(listen_ep);
+        }
+
+        sched.schedule();
+
+        // Server woke up with incoming message delivered
+        if let Some(srv) = sched.get_thread_mut(server_tid) {
+            let sender_tid = srv.caller_tid;
+            let opcode = srv.ipc_buffer.opcode;
+            let a0 = srv.ipc_buffer.args[0];
+            let a1 = srv.ipc_buffer.args[1];
+            return Ok((sender_tid, opcode, a0, a1));
+        }
+
+        Ok((0, 0, 0, 0))
+    }
+}
+
+pub fn init() {
+    unsafe {
+        (*core::ptr::addr_of_mut!(EP_MANAGER)).init();
+        kprintln!("[IPC] Synchronous Rendezvous Engine initialized ({} Endpoints).", MAX_ENDPOINTS);
+    }
+}

kernel/src/ipc/mod.rs

@@ -1,5 +1,7 @@
 pub mod fastpath;
+pub mod endpoint;
 
 pub fn init() {
-    crate::kprintln!("[IPC] Capability-based Fast-Path IPC Ready.");
+    endpoint::init();
+    crate::kprintln!("[IPC] Capability-based Fast-Path & Rendezvous IPC Ready.");
 }

kernel/src/main.rs

@@ -11,26 +11,17 @@ pub mod loader;
 
 use core::panic::PanicInfo;
 use limine_requests::*;
+use sched::SCHEDULER;
 
 // Include assembly trampolines directly into the binary
 core::arch::global_asm!(include_str!("../asm/context.S"));
 core::arch::global_asm!(include_str!("../asm/syscall_entry.S"));
 core::arch::global_asm!(include_str!("../asm/ring3_enter.S"));
 
-extern "C" {
-    fn enter_user_mode(
-        entry_rip: u64,
-        user_rsp: u64,
-        pml4_paddr: u64,
-        kernel_stack_top: u64,
-    ) -> !;
-}
-
 #[no_mangle]
 pub extern "C" fn _start() -> ! {
     // 1. Validate Limine Base Revision
     if BASE_REVISION.revision != 3 {
-        // Revision mismatch
         loop { core::hint::spin_loop(); }
     }
 
@@ -84,29 +75,44 @@ pub extern "C" fn _start() -> ! {
     );
     kprintln!("[TEST] IPC Fast-Path Ping Result: status={}, opcode={:#x}, val={}", status, resp_op, val);
 
-    // 9. Inspect and Load Initial Userspace Process (Ring 3 Transition)
+    // 9. Inspect and Load Initial Userspace Processes
     unsafe {
         let mod_resp = MODULE_REQUEST.response;
         if !mod_resp.is_null() && (*mod_resp).module_count > 0 {
-            kprintln!("[BOOT] Loading root userspace module [0] into Ring 3...");
+            let count = (*mod_resp).module_count;
-            let mod_file = *(*mod_resp).modules;
+            kprintln!("[BOOT] Detected {} userspace boot modules. Creating tasks...", count);
-            let elf_slice = core::slice::from_raw_parts((*mod_file).address, (*mod_file).size as usize);
 
-            match loader::elf::load_elf(elf_slice, hhdm_offset) {
+            let sched_ptr = core::ptr::addr_of_mut!(SCHEDULER);
-                Ok(proc) => {
+
-                    kprintln!("[BOOT] Transitioning CPU to Ring 3 (User Mode) via IRETQ...");
+            for i in 0..count {
-                    kprintln!("-------------------------------------------------------");
+                let mod_file = *(*mod_resp).modules.add(i as usize);
-                    enter_user_mode(
+                let elf_slice = core::slice::from_raw_parts((*mod_file).address, (*mod_file).size as usize);
-                        proc.entry_rip,
+                let name = match i {
-                        proc.user_rsp,
+                    0 => "init_server",
-                        proc.pml4_paddr,
+                    1 => "uart_driver",
-                        proc.kernel_stack_top,
+                    _ => "user_server",
-                    );
+                };
-                }
+
-                Err(err) => {
+                match loader::elf::load_elf(elf_slice, hhdm_offset) {
-                    kprintln!("[ERROR] Failed to load ELF process: {}", err);
+                    Ok(proc) => {
+                        let tid = (*sched_ptr).create_user_thread(
+                            name,
+                            proc.entry_rip,
+                            proc.user_rsp,
+                            proc.pml4_paddr,
+                            proc.kernel_stack_top,
+                        );
+                        kprintln!("[BOOT] Registered Thread '{}' with TID {:?}", name, tid);
+                    }
+                    Err(err) => {
+                        kprintln!("[ERROR] Failed to load module [{}]: {}", i, err);
+                    }
                 }
             }
+
+            kprintln!("[BOOT] Starting Multi-Tasking & IPC Rendezvous...");
+            kprintln!("-------------------------------------------------------");
+            (*sched_ptr).schedule();
         } else {
             kprintln!("[BOOT] No userspace modules detected. Staying in Ring 0 idle loop.");
         }

kernel/src/sched/mod.rs

@@ -1,35 +1,212 @@
-//! Minimal Thread & Task Scheduler Subsystem
+//! Preemptive & Cooperative Microkernel Scheduler
 
-#[repr(C)]
+pub mod thread;
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+
-pub enum ThreadState {
+use thread::{Thread, ThreadState, MAX_THREADS};
-    Ready,
+use crate::arch::gdt::set_kernel_stack;
-    Running,
+use crate::mm::vmm::VMM;
-    BlockedOnIpc,
+use crate::kprintln;
-    Dead,
+
+extern "C" {
+    fn switch_to(prev_rsp: *mut u64, next_rsp: u64);
 }
 
-#[repr(C)]
+pub struct Scheduler {
-pub struct ContextFrame {
+    pub threads: [Thread; MAX_THREADS],
-    pub r15: u64,
+    pub current_tid: usize,
-    pub r14: u64,
+    pub next_tid: u64,
-    pub r13: u64,
-    pub r12: u64,
-    pub rbx: u64,
-    pub rbp: u64,
-    pub rsp: u64,
-    pub rip: u64,
-    pub rflags: u64,
 }
 
-pub struct Thread {
+pub static mut SCHEDULER: Scheduler = Scheduler {
-    pub id: u64,
+    threads: [const { Thread::empty() }; MAX_THREADS],
-    pub state: ThreadState,
+    current_tid: 0,
-    pub rsp: u64,
+    next_tid: 1,
-    pub cr3: u64,
+};
-    pub kernel_stack_top: u64,
+
+impl Scheduler {
+    pub fn init(&mut self) {
+        // Slot 0 is reserved for Kernel Idle/Bootstrap thread
+        self.threads[0].id = 0;
+        self.threads[0].name = "kernel_idle";
+        self.threads[0].state = ThreadState::Running;
+        self.current_tid = 0;
+        self.next_tid = 1;
+        kprintln!("[SCHED] Scheduler initialized (Multi-Thread TCB table ready).");
+    }
+
+    pub fn create_user_thread(
+        &mut self,
+        name: &'static str,
+        entry_rip: u64,
+        user_rsp: u64,
+        pml4_paddr: u64,
+        kernel_stack_top: u64,
+    ) -> Option<u64> {
+        for i in 1..MAX_THREADS {
+            if self.threads[i].state == ThreadState::Unused {
+                let tid = self.next_tid;
+                self.next_tid += 1;
+
+                self.threads[i].id = tid;
+                self.threads[i].name = name;
+                self.threads[i].state = ThreadState::Ready;
+                self.threads[i].pml4_paddr = pml4_paddr;
+                self.threads[i].kernel_stack_top = kernel_stack_top;
+                self.threads[i].user_rsp = user_rsp;
+                self.threads[i].user_rip = entry_rip;
+
+                // Setup initial kernel stack for IRETQ return
+                unsafe {
+                    let kstack = kernel_stack_top as *mut u64;
+                    let stack_ptr = kstack.sub(16);
+                    
+                    *stack_ptr.add(0) = 0; // r15
+                    *stack_ptr.add(1) = 0; // r14
+                    *stack_ptr.add(2) = 0; // r13
+                    *stack_ptr.add(3) = 0; // r12
+                    *stack_ptr.add(4) = 0; // rbx
+                    *stack_ptr.add(5) = 0; // rbp
+                    *stack_ptr.add(6) = initial_user_trampoline as *const () as usize as u64; // RIP for switch_to
+
+                    self.threads[i].context.rsp = stack_ptr as u64;
+                }
+
+                kprintln!("[SCHED] Created User Thread [TID {}] '{}' (RIP: {:#x}, RSP: {:#x})",
+                    tid, name, entry_rip, user_rsp);
+                return Some(tid);
+            }
+        }
+        None
+    }
+
+    pub fn get_current_mut(&mut self) -> &mut Thread {
+        &mut self.threads[self.current_tid]
+    }
+
+    pub fn get_thread_mut(&mut self, tid: u64) -> Option<&mut Thread> {
+        for t in self.threads.iter_mut() {
+            if t.id == tid && t.state != ThreadState::Unused {
+                return Some(t);
+            }
+        }
+        None
+    }
+
+    pub fn block_current(&mut self, new_state: ThreadState) {
+        self.threads[self.current_tid].state = new_state;
+        self.schedule();
+    }
+
+    pub fn unblock(&mut self, tid: u64) {
+        if let Some(t) = self.get_thread_mut(tid) {
+            if t.state != ThreadState::Unused && t.state != ThreadState::Dead {
+                t.state = ThreadState::Ready;
+            }
+        }
+    }
+
+    pub fn yield_current(&mut self) {
+        if self.threads[self.current_tid].state == ThreadState::Running {
+            self.threads[self.current_tid].state = ThreadState::Ready;
+        }
+        self.schedule();
+    }
+
+    pub fn schedule(&mut self) {
+        let prev_idx = self.current_tid;
+        let mut next_idx = (prev_idx + 1) % MAX_THREADS;
+
+        // Find next Ready thread (Round-Robin)
+        let mut found = false;
+        for _ in 0..MAX_THREADS {
+            if self.threads[next_idx].state == ThreadState::Ready {
+                found = true;
+                break;
+            }
+            next_idx = (next_idx + 1) % MAX_THREADS;
+        }
+
+        if !found {
+            if self.threads[prev_idx].state == ThreadState::Running {
+                return;
+            }
+            next_idx = 0; // idle thread
+        }
+
+        if prev_idx == next_idx && self.threads[prev_idx].state == ThreadState::Running {
+            return;
+        }
+
+        if self.threads[prev_idx].state == ThreadState::Running {
+            self.threads[prev_idx].state = ThreadState::Ready;
+        }
+
+        self.current_tid = next_idx;
+        self.threads[next_idx].state = ThreadState::Running;
+
+        let next_pml4 = self.threads[next_idx].pml4_paddr;
+        let next_kstack = self.threads[next_idx].kernel_stack_top;
+        let next_rsp = self.threads[next_idx].context.rsp;
+        let prev_rsp_ptr = core::ptr::addr_of_mut!(self.threads[prev_idx].context.rsp);
+
+        unsafe {
+            if next_pml4 != 0 {
+                (*core::ptr::addr_of_mut!(VMM)).load_cr3(next_pml4);
+            }
+            if next_kstack != 0 {
+                set_kernel_stack(next_kstack);
+            }
+            switch_to(prev_rsp_ptr, next_rsp);
+        }
+    }
 }
 
 pub fn init() {
-    crate::kprintln!("[SCHED] Scheduler initialized (Round-Robin preemptive stub).");
+    unsafe {
+        (*core::ptr::addr_of_mut!(SCHEDULER)).init();
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn initial_user_trampoline() {
+    let (entry_rip, user_rsp) = unsafe {
+        let sched = &mut *core::ptr::addr_of_mut!(SCHEDULER);
+        let curr = sched.get_current_mut();
+        (curr.user_rip, curr.user_rsp)
+    };
+
+    unsafe {
+        // IRETQ to Ring 3
+        core::arch::asm!(
+            "mov ax, 0x1B",
+            "mov ds, ax",
+            "mov es, ax",
+            "mov fs, ax",
+            "mov gs, ax",
+            "push 0x1B",          // User SS
+            "push {user_rsp}",    // User RSP
+            "push 0x0202",        // RFLAGS (IF enabled)
+            "push 0x23",          // User CS
+            "push {entry_rip}",   // User RIP
+            "xor rax, rax",
+            "xor rbx, rbx",
+            "xor rcx, rcx",
+            "xor rdx, rdx",
+            "xor rsi, rsi",
+            "xor rdi, rdi",
+            "xor rbp, rbp",
+            "xor r8, r8",
+            "xor r9, r9",
+            "xor r10, r10",
+            "xor r11, r11",
+            "xor r12, r12",
+            "xor r13, r13",
+            "xor r14, r14",
+            "xor r15, r15",
+            "iretq",
+            user_rsp = in(reg) user_rsp,
+            entry_rip = in(reg) entry_rip,
+            options(noreturn)
+        );
+    }
 }

kernel/src/sched/thread.rs

@@ -0,0 +1,79 @@
+//! Thread Control Block (TCB) & Context Definition
+
+use crate::ipc::fastpath::FastPathMessage;
+
+pub const MAX_THREADS: usize = 16;
+pub const KERNEL_STACK_SIZE: usize = 16384; // 16 KiB
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ThreadState {
+    Unused,
+    Ready,
+    Running,
+    BlockedOnSend(u64),  // Waiting on destination endpoint ID
+    BlockedOnRecv(u64),  // Waiting for messages on endpoint ID
+    BlockedOnReply(u64), // Client waiting for specific server TID reply
+    Dead,
+}
+
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub struct SavedContext {
+    pub r15: u64,
+    pub r14: u64,
+    pub r13: u64,
+    pub r12: u64,
+    pub rbx: u64,
+    pub rbp: u64,
+    pub rsp: u64, // Saved kernel RSP
+}
+
+impl SavedContext {
+    pub const fn empty() -> Self {
+        Self {
+            r15: 0,
+            r14: 0,
+            r13: 0,
+            r12: 0,
+            rbx: 0,
+            rbp: 0,
+            rsp: 0,
+        }
+    }
+}
+
+pub struct Thread {
+    pub id: u64,
+    pub name: &'static str,
+    pub state: ThreadState,
+    pub context: SavedContext,
+    pub pml4_paddr: u64,
+    pub kernel_stack_base: u64,
+    pub kernel_stack_top: u64,
+    pub user_rsp: u64,
+    pub user_rip: u64,
+    pub ipc_buffer: FastPathMessage,
+    pub caller_tid: u64, // TID of client waiting for reply
+}
+
+impl Thread {
+    pub const fn empty() -> Self {
+        Self {
+            id: 0,
+            name: "",
+            state: ThreadState::Unused,
+            context: SavedContext::empty(),
+            pml4_paddr: 0,
+            kernel_stack_base: 0,
+            kernel_stack_top: 0,
+            user_rsp: 0,
+            user_rip: 0,
+            ipc_buffer: FastPathMessage {
+                dest_cap: 0,
+                opcode: 0,
+                args: [0; 4],
+            },
+            caller_tid: 0,
+        }
+    }
+}

lib/libipc_cpp/include/ipc.hpp

@@ -24,6 +24,13 @@ struct FastResponse {
     uint64_t out1;
 };
 
+struct IncomingRequest {
+    uint64_t sender_tid;
+    uint64_t opcode;
+    uint64_t arg0;
+    uint64_t arg1;
+};
+
 // RAII Capability Handle
 class Capability {
 private:
@@ -56,7 +63,7 @@ public:
     [[nodiscard]] constexpr bool is_valid() const noexcept { return handle_ != CAP_NULL; }
 };
 
-// RAII IPC Endpoint Wrapper
+// RAII IPC Client Endpoint Wrapper
 class Endpoint {
 private:
     Capability cap_;
@@ -82,6 +89,36 @@ public:
     [[nodiscard]] cap_t raw_handle() const noexcept { return cap_.get(); }
 };
 
+// RAII IPC Server Endpoint Wrapper
+class ServerEndpoint {
+private:
+    Capability cap_;
+
+public:
+    explicit ServerEndpoint(Capability&& cap) noexcept : cap_(static_cast<Capability&&>(cap)) {}
+    explicit ServerEndpoint(cap_t cap) noexcept : cap_(cap) {}
+
+    // Server main loop: Reply to previous sender and wait for next request
+    [[nodiscard]] Result<IncomingRequest> reply_and_recv(
+        uint64_t reply_to_tid = 0,
+        uint64_t reply_val0 = 0,
+        uint64_t reply_val1 = 0
+    ) const noexcept {
+        uint64_t sender_tid = 0;
+        uint64_t opcode = 0;
+        uint64_t arg0 = 0;
+        uint64_t arg1 = 0;
+        sysret_t ret = sys_ipc_reply_recv_raw(cap_.get(), reply_to_tid, reply_val0, reply_val1,
+                                              &sender_tid, &opcode, &arg0, &arg1);
+        return Result<IncomingRequest>{
+            .status = ret,
+            .value = { sender_tid, opcode, arg0, arg1 }
+        };
+    }
+
+    [[nodiscard]] cap_t raw_handle() const noexcept { return cap_.get(); }
+};
+
 // RAII Shared Memory Buffer
 class SharedBuffer {
 private:

servers/init/main.cpp

@@ -2,29 +2,34 @@
 #include "stdio.h"
 #include "string.h"
 
+#define UART_SERVICE_ENDPOINT 2
+
 extern "C" void _start() {
     puts("[INIT-SERVER] Userspace Root Init Server Started (Ring 3)!");
 
-    // 1. Instantiate RAII IPC Endpoint for Kernel Control
+    // 1. Communicate with Kernel Control (Endpoint 1)
-    ipc::Endpoint kernel_ctrl(CAP_SELF_TASK);
+    ipc::Endpoint kernel_ctrl(CAP_KERNEL_CONTROL);
+    puts("[INIT-SERVER] Sending Fast-Path IPC Ping to Microkernel (CAP 1)...");
+    auto kping = kernel_ctrl.call(IPC_OP_PING, 777);
+    if (kping.is_ok()) {
+        puts("[INIT-SERVER] Received Pong from Microkernel!");
+    }
 
-    puts("[INIT-SERVER] Sending Fast-Path IPC Ping to Microkernel...");
+    // 2. Communicate with UART Driver Service (Endpoint 2) via Rendezvous IPC
-    auto ping_res = kernel_ctrl.call(IPC_OP_PING, 100);
+    ipc::Endpoint uart_service(UART_SERVICE_ENDPOINT);
+    puts("[INIT-SERVER] Sending Synchronous Rendezvous IPC to UART Driver (Endpoint 2)...");
 
-    if (ping_res.is_ok()) {
+    // Pack 4 characters 'O', 'K', '!', '\n' into 32-bit arg0
-        puts("[INIT-SERVER] Received IPC Ping Response from Microkernel!");
+    uint64_t msg_chars = 'O' | ('K' << 8) | ('!' << 16) | ('\n' << 24);
+    auto uart_res = uart_service.call(IPC_OP_WRITE, msg_chars);
+
+    if (uart_res.is_ok()) {
+        puts("[INIT-SERVER] Rendezvous IPC Call to UART Driver SUCCEEDED!");
     } else {
-        puts("[INIT-SERVER] IPC Call Failed!");
+        puts("[INIT-SERVER] Rendezvous IPC Call failed.");
     }
 
-    // 2. Register Server Endpoint
+    puts("[INIT-SERVER] Root Init Server running. Entering worker loop.");
-    puts("[INIT-SERVER] Registering Root Server Capability...");
-    auto reg_res = kernel_ctrl.call(IPC_OP_REGISTER_SERVER, 0x1337BEEF);
-    if (reg_res.is_ok()) {
-        puts("[INIT-SERVER] Server Registered Successfully!");
-    }
-
-    puts("[INIT-SERVER] Root Init Server ready. Entering event loop.");
 
     while (true) {
         sys_yield();

servers/uart_driver/main.c

@@ -1,17 +1,63 @@
 #include "uart.h"
 #include "stdio.h"
 #include "abi/syscalls.h"
+#include "abi/ipc.h"
+
+#define UART_SERVICE_ENDPOINT 2
 
 extern void _start(void) {
     uart_init(UART_COM1_PORT, 115200);
-    uart_puts(UART_COM1_PORT, "[RING3-UART-DRIVER] 16550 UART COM1 initialized from userspace!\n");
+    puts("[UART-DRIVER] 16550 UART Server started in Ring 3. Listening for IPC requests...");
+
+    uint64_t last_client = 0;
+    uint64_t reply_val0 = 0;
+    uint64_t reply_val1 = 0;
 
     while (1) {
-        if (uart_has_data(UART_COM1_PORT)) {
+        uint64_t sender_tid = 0;
-            char c = uart_getc(UART_COM1_PORT);
+        uint64_t opcode = 0;
-            // Echo back
+        uint64_t arg0 = 0;
-            uart_putc(UART_COM1_PORT, c);
+        uint64_t arg1 = 0;
+
+        // Block and wait for next client IPC request, replying to last client if any
+        sysret_t ret = sys_ipc_reply_recv_raw(
+            UART_SERVICE_ENDPOINT,
+            last_client,
+            reply_val0,
+            reply_val1,
+            &sender_tid,
+            &opcode,
+            &arg0,
+            &arg1
+        );
+
+        if (ret == 0 && sender_tid != 0) {
+            last_client = sender_tid;
+
+            if (opcode == IPC_OP_WRITE) {
+                // Write received characters to UART hardware
+                char c0 = (char)(arg0 & 0xFF);
+                char c1 = (char)((arg0 >> 8) & 0xFF);
+                char c2 = (char)((arg0 >> 16) & 0xFF);
+                char c3 = (char)((arg0 >> 24) & 0xFF);
+
+                if (c0) uart_putc(UART_COM1_PORT, c0);
+                if (c1) uart_putc(UART_COM1_PORT, c1);
+                if (c2) uart_putc(UART_COM1_PORT, c2);
+                if (c3) uart_putc(UART_COM1_PORT, c3);
+
+                reply_val0 = 0; // Success
+                reply_val1 = 4; // Bytes written
+            } else if (opcode == IPC_OP_PING) {
+                reply_val0 = IPC_OP_PONG;
+                reply_val1 = arg0 + 1;
+            } else {
+                reply_val0 = (uint64_t)-1;
+                reply_val1 = 0;
+            }
+        } else {
+            last_client = 0;
+            sys_yield();
         }
-        sys_yield();
     }
 }