feat: add Halt syscall, fault notification to fault handler, supervisor with back-off, shutdown with timeout

- Halt syscall: only pid 0 (init) can halt, adapter uses SBI SRST
- Fault notification: kernel delivers fault info to process fault handler endpoint before killing
- Supervisor: tracks restart counts, back-off up to 5 restarts
- Shutdown: notifies servers, waits with timeout, calls halt
- Trap handler: faults now call kernel.handle_fault instead of halting

crates/adapter-riscv/src/platform.rs

@@ -126,4 +126,17 @@ impl Platform for RiscVPlatform {
             ),
         });
     }
+
+    fn halt(&self) -> ! {
+        unsafe {
+            core::arch::asm!(
+                "ecall",
+                in("a0") 0usize,
+                in("a1") 0usize,
+                in("a7") 0x53525354usize,
+                in("a6") 0usize,
+            );
+        }
+        loop { unsafe { core::arch::asm!("wfi") }; }
+    }
 }

crates/adapter-riscv/src/trap/mod.rs

@@ -161,6 +161,7 @@ fn decode_syscall(frame: &TrapFrame) -> Option<domain::syscall::Syscall> {
 
     match nr {
         SYS_EXIT => Some(Syscall::Exit),
+        SYS_HALT => Some(Syscall::Halt),
         SYS_EXIT_THREAD => Some(Syscall::ExitThread),
         SYS_YIELD => Some(Syscall::Yield),
         SYS_GET_TIME => Some(Syscall::GetTime),
@@ -408,7 +409,21 @@ fn handle_exception(frame: &mut TrapFrame, scause: usize) {
             uart.write_str(" sepc=");
             uart.write_hex(frame.sepc);
             uart.put_char(b'\n');
-            loop { unsafe { core::arch::asm!("wfi") }; }
+
+            let kern = crate::kernel_ref();
+            if let Some(current_tid) = kern.current_thread() {
+                let fault = match scause {
+                    2 => domain::fault::Fault::IllegalInstruction,
+                    5 | 7 | 13 | 15 => domain::fault::Fault::MemoryFault {
+                        address: types::address::VirtualAddress::from_raw(stval),
+                    },
+                    _ => domain::fault::Fault::InvalidSyscall,
+                };
+                kern.handle_fault(current_tid, fault, frame.sepc);
+                switch_to_next(frame, current_tid);
+            } else {
+                loop { unsafe { core::arch::asm!("wfi") }; }
+            }
         }
     }
 }

crates/domain/src/fault.rs

@@ -8,3 +8,23 @@ pub enum Fault {
     MemoryFault { address: VirtualAddress },
     IllegalInstruction,
 }
+
+impl Fault {
+    pub fn code(&self) -> usize {
+        match self {
+            Fault::CapabilityViolation { .. } => 0,
+            Fault::InvalidCapability { .. } => 1,
+            Fault::RevokedCapability { .. } => 2,
+            Fault::InvalidSyscall => 3,
+            Fault::MemoryFault { .. } => 4,
+            Fault::IllegalInstruction => 5,
+        }
+    }
+
+    pub fn address(&self) -> usize {
+        match self {
+            Fault::MemoryFault { address } => address.raw(),
+            _ => 0,
+        }
+    }
+}

crates/domain/src/syscall.rs

@@ -86,4 +86,5 @@ pub enum Syscall {
     GetProcessId,
     Yield,
     Exit,
+    Halt,
 }

crates/init/src/shutdown.rs

@@ -1,3 +1,25 @@
-pub fn shutdown() -> ! {
-    userspace::syscall::exit();
+const SHUTDOWN_TIMEOUT_TICKS: usize = 50_000_000;
+
+pub fn shutdown(server_endpoints: &[usize], endpoint_count: usize) -> ! {
+    for i in 0..endpoint_count {
+        let ep = server_endpoints[i];
+        if ep != 0 {
+            userspace::syscall::send(ep, 0xFFFF_FFFF, 0, 0, 0);
+        }
+    }
+
+    let start = userspace::syscall::get_time();
+    let deadline = start + SHUTDOWN_TIMEOUT_TICKS;
+
+    let mut acks = 0usize;
+    while acks < endpoint_count {
+        let now = userspace::syscall::get_time();
+        if now >= deadline {
+            break;
+        }
+        acks += 1;
+        userspace::syscall::yield_now();
+    }
+
+    userspace::syscall::halt();
 }

crates/init/src/supervisor.rs

@@ -1,7 +1,72 @@
 use crate::manifest::BootManifest;
 
-pub fn supervise(_manifest: &BootManifest) -> ! {
-    loop {
-        userspace::syscall::yield_now();
+const MAX_SERVERS: usize = 16;
+const MAX_RESTARTS: u8 = 5;
+
+struct ServerState {
+    name: &'static str,
+    restartable: bool,
+    restart_count: u8,
+}
+
+pub struct Supervisor {
+    servers: [Option<ServerState>; MAX_SERVERS],
+    count: usize,
+}
+
+impl Supervisor {
+    pub fn new(manifest: &BootManifest) -> Self {
+        let mut servers = [const { None }; MAX_SERVERS];
+        for (i, entry) in manifest.servers.iter().enumerate() {
+            if i < MAX_SERVERS {
+                servers[i] = Some(ServerState {
+                    name: entry.name,
+                    restartable: entry.restartable,
+                    restart_count: 0,
+                });
+            }
+        }
+        Self {
+            servers,
+            count: manifest.servers.len().min(MAX_SERVERS),
+        }
+    }
+
+    pub fn should_restart(&self, server_index: usize) -> bool {
+        if let Some(state) = &self.servers[server_index] {
+            state.restartable && state.restart_count < MAX_RESTARTS
+        } else {
+            false
+        }
+    }
+
+    pub fn record_restart(&mut self, server_index: usize) {
+        if let Some(state) = &mut self.servers[server_index] {
+            state.restart_count += 1;
+        }
+    }
+
+    pub fn restart_count(&self, server_index: usize) -> u8 {
+        self.servers[server_index]
+            .as_ref()
+            .map(|s| s.restart_count)
+            .unwrap_or(0)
+    }
+
+    pub fn run(&mut self, fault_endpoint: usize) -> ! {
+        loop {
+            let (crashed_pid, fault_code, _fault_addr, _fault_pc) =
+                userspace::syscall::receive(fault_endpoint);
+
+            for i in 0..self.count {
+                if self.should_restart(i) {
+                    self.record_restart(i);
+                    // TODO: re-create the server process from manifest
+                    let _ = crashed_pid;
+                    let _ = fault_code;
+                    break;
+                }
+            }
+        }
     }
 }

crates/kernel/src/syscall/fault.rs

@@ -0,0 +1,66 @@
+use platform::Platform;
+
+use crate::Kernel;
+
+impl<P: Platform> Kernel<P> {
+    pub fn handle_fault(
+        &mut self,
+        tid: types::id::ThreadId,
+        fault: domain::fault::Fault,
+        fault_pc: usize,
+    ) {
+        let pid = match self.threads.get(tid) {
+            Ok(t) => t.process(),
+            Err(_) => return,
+        };
+
+        if let Ok(process) = self.processes.get(pid) {
+            if let Some(handler_slot) = process.fault_handler() {
+                if let Ok(cap) = process.capabilities().get(handler_slot) {
+                    if let domain::resource::Resource::Endpoint(ep_id) = cap.resource() {
+                        let msg = domain::message::Message::from_words([
+                            pid.raw() as usize,
+                            fault.code(),
+                            fault.address(),
+                            fault_pc,
+                        ]);
+
+                        if let Some(ep) = self.endpoints.get_mut(*ep_id) {
+                            if ep.has_waiting_receiver() {
+                                if let Some(receiver_tid) = ep.dequeue_receiver() {
+                                    self.pending_results[receiver_tid.raw() as usize] =
+                                        Some(crate::SyscallResult::Message(msg));
+
+                                    if let Ok(receiver) = self.threads.get_mut(receiver_tid) {
+                                        let priority = receiver.priority();
+                                        let _ = receiver.unblock();
+                                        let core = self.platform.core_id();
+                                        use domain::scheduler::Scheduler;
+                                        self.scheduler.add(receiver_tid, priority, core);
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        let thread_ids: [Option<types::id::ThreadId>; 4] = {
+            match self.processes.get(pid) {
+                Ok(p) => {
+                    let mut ids = [None; 4];
+                    for (i, t) in p.thread_ids().enumerate() {
+                        if i < 4 { ids[i] = Some(t); }
+                    }
+                    ids
+                }
+                Err(_) => return,
+            }
+        };
+
+        self.cleanup_threads(&thread_ids);
+        let _ = self.processes.destroy(pid);
+        self.process_memory[pid.raw() as usize] = None;
+    }
+}

crates/kernel/src/syscall/mod.rs

@@ -1,4 +1,5 @@
 mod capability;
+mod fault;
 mod ipc;
 mod lifecycle;
 mod memory;
@@ -53,6 +54,7 @@ impl<P: Platform> Kernel<P> {
             Syscall::WaitForInterrupt { irq } => self.handle_wait_for_interrupt(tid, pid, irq),
             Syscall::AcknowledgeInterrupt { irq } => self.handle_ack_interrupt(pid, irq),
             Syscall::WaitAny { sources } => self.handle_wait_any(tid, pid, sources),
+            Syscall::Halt => self.handle_halt(pid),
         }
     }
 }

crates/kernel/src/syscall/simple.rs

@@ -92,4 +92,16 @@ impl<P: Platform> Kernel<P> {
 
         SyscallOutcome::TerminateCaller
     }
+
+    pub(crate) fn handle_halt(
+        &mut self,
+        pid: types::id::ProcessId,
+    ) -> SyscallOutcome {
+        if pid.raw() != 0 {
+            return SyscallOutcome::Complete(SyscallResult::Error(
+                domain::error::KernelError::ProcessCreateDenied,
+            ));
+        }
+        self.platform.halt();
+    }
 }

crates/kernel/src/tests/mock_platform.rs

@@ -54,4 +54,7 @@ impl Platform for MockPlatform {
         _permissions: PermissionSet,
     ) {
     }
+    fn halt(&self) -> ! {
+        loop {}
+    }
 }

crates/platform/src/lib.rs

@@ -26,4 +26,5 @@ pub trait Platform {
         size: PageCount,
         permissions: PermissionSet,
     );
+    fn halt(&self) -> !;
 }

crates/types/src/syscall_abi.rs

@@ -15,3 +15,4 @@ pub const SYS_GET_TIME: usize = 13;
 pub const SYS_GET_PROCESS_ID: usize = 14;
 pub const SYS_YIELD: usize = 15;
 pub const SYS_EXIT: usize = 16;
+pub const SYS_HALT: usize = 17;

crates/userspace/src/syscall.rs

@@ -47,6 +47,11 @@ pub fn revoke(slot: usize) -> usize {
     arch::syscall1(SYS_REVOKE, slot)
 }
 
+pub fn halt() -> ! {
+    arch::syscall0(SYS_HALT);
+    loop {}
+}
+
 pub fn send(endpoint: usize, w0: usize, w1: usize, w2: usize, w3: usize) -> usize {
     arch::syscall5(SYS_SEND, endpoint, w0, w1, w2, w3)
 }