feat: SMP stage 2, lock-free fast paths (GetTime, GetProcessId, DebugPutchar, no-op Yield), storm checks yield equivalence

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

@@ -241,7 +241,7 @@ fn next_runnable() -> types::id::ThreadId {
             return tid;
         }
         crate::mmu::activate(crate::mmu::page_table::table_at(crate::smp::kernel_root()));
-        crate::smp::unlock();
+        release();
         unsafe { core::arch::asm!("wfi") };
         crate::smp::service_ipi();
         crate::smp::lock();
@@ -271,6 +271,54 @@ fn dispatch_external() {
     }
 }
 
+/// Releases the kernel lock, first publishing what the lock-free fast
+/// paths may rely on.
+fn release() {
+    let kern = kern();
+    for core in 0..crate::smp::num_harts() {
+        let id = types::id::CoreId::from_raw(core as u8);
+        crate::smp::publish_waiting(core, kern.highest_waiting(id).map(|p| p.value()));
+    }
+    let running = kern.current_thread();
+    crate::smp::publish_running(
+        running
+            .and_then(|t| kern.thread_process(t))
+            .map(|p| p.raw()),
+        running
+            .and_then(|t| kern.thread_priority(t))
+            .map(|p| p.value()),
+    );
+    crate::smp::unlock();
+}
+
+/// Syscalls answered without the kernel lock, as the kernel would answer
+/// them: they read nothing the lock guards (the time, the UART, the
+/// running thread's fixed process id) or, for a Yield that has nobody to
+/// yield to, change nothing. Any pending IPI or reschedule request sends
+/// the call down the slow path, so no request is ever skipped.
+fn fast_syscall(frame: &mut TrapFrame) -> bool {
+    use types::syscall_abi::{SYS_DEBUG_PUTCHAR, SYS_GET_PROCESS_ID, SYS_GET_TIME, SYS_YIELD};
+    if crate::smp::has_pending_request() {
+        return false;
+    }
+    let result = match frame.regs[17] {
+        SYS_GET_TIME => kernel::SyscallResult::Value(crate::hw::timer::read_time() as usize),
+        SYS_GET_PROCESS_ID => match crate::smp::running_pid() {
+            Some(pid) => kernel::SyscallResult::Value(pid),
+            None => return false,
+        },
+        SYS_DEBUG_PUTCHAR => {
+            crate::hw::uart::Uart::new().put_char(frame.regs[10] as u8);
+            kernel::SyscallResult::Ok
+        }
+        SYS_YIELD if crate::smp::yield_is_noop() => kernel::SyscallResult::Ok,
+        _ => return false,
+    };
+    frame.sepc += 4;
+    write_result(frame, &result);
+    true
+}
+
 /// Last step before leaving the kernel to userspace: act on a reschedule
 /// request, make sure this hart has a thread (its own may have been taken
 /// away by another hart), and translate through that thread's address
@@ -311,7 +359,7 @@ pub fn enter_first() -> ! {
         None => switch_to_next(&mut frame, None),
     }
     finish(&mut frame);
-    crate::smp::unlock();
+    release();
     // SAFETY: `frame` holds a user context; sscratch is set from it on
     // the way out, so later traps land at the top of this frame.
     unsafe { trap_return(&mut frame) }
@@ -326,6 +374,9 @@ extern "C" fn trap_handler_rust(frame: &mut TrapFrame) {
         // Checked before taking the lock: this hart may already hold it.
         kernel_fault(frame, scause);
     }
+    if scause == CAUSE_ECALL_U && fast_syscall(frame) {
+        return;
+    }
     crate::smp::service_ipi();
     crate::smp::lock();
     if scause & CAUSE_INTERRUPT_BIT != 0 {
@@ -334,7 +385,7 @@ extern "C" fn trap_handler_rust(frame: &mut TrapFrame) {
         handle_exception(frame, scause);
     }
     finish(frame);
-    crate::smp::unlock();
+    release();
 }
 
 fn handle_interrupt(frame: &mut TrapFrame, cause: usize) {