feat(kernel): WatchDeath + SetTimer events as notification signals; adapter wakes at earliest deadline

crates/adapter-riscv/src/hw/timer.rs

@@ -1,4 +1,10 @@
+use core::sync::atomic::{AtomicU64, Ordering};
+
 use crate::board;
+use crate::smp::{MAX_HARTS, hart_id};
+
+/// When each hart's timer is set to go off next.
+static NEXT: [AtomicU64; MAX_HARTS] = [const { AtomicU64::new(u64::MAX) }; MAX_HARTS];
 
 pub fn read_time() -> u64 {
     let time: u64;
@@ -21,15 +27,27 @@ fn sbi_set_timer(stime: u64) {
 }
 
 pub fn init() {
-    let now = read_time();
-    sbi_set_timer(now + board::TICK_INTERVAL);
+    set_next(read_time() + board::TICK_INTERVAL);
 }
 
 pub fn set_next(deadline: u64) {
+    if let Some(next) = NEXT.get(hart_id()) {
+        next.store(deadline, Ordering::Relaxed);
+    }
     sbi_set_timer(deadline);
 }
 
-pub fn handle_interrupt() {
-    let now = read_time();
-    sbi_set_timer(now + board::TICK_INTERVAL);
+/// Goes off at `deadline` too, if that is before the next tick.
+pub fn wake_at(deadline: u64) {
+    if NEXT
+        .get(hart_id())
+        .is_some_and(|n| deadline < n.load(Ordering::Relaxed))
+    {
+        set_next(deadline);
+    }
+}
+
+/// Re-arms the tick; the kernel then re-arms for its earliest timer.
+pub fn handle_interrupt() {
+    set_next(read_time() + board::TICK_INTERVAL);
 }