feat(kernel): console cap gates DebugPutchar, no uart fast path; spawn start arg in a0; delegate returns slot

crates/types/src/bootinfo.rs

@@ -34,6 +34,7 @@ pub struct BootInfo {
     pub magic: u64,
     pub device_count: u64,
     pub system_control_slot: u64,
+    pub console_slot: u64,
     pub devices: [BootDevice; MAX_BOOT_DEVICES],
 }
 

crates/types/src/lib.rs

@@ -12,6 +12,7 @@ pub mod mapping;
 pub mod permission;
 #[cfg(kani)]
 pub mod proofs;
+pub mod start_arg;
 pub mod storage;
 pub mod syscall_abi;
 pub mod time;

crates/types/src/start_arg.rs

@@ -0,0 +1,43 @@
+//! The word a process's first thread gets in `a0`: the slots of the
+//! capabilities any process may be handed at start (console, log endpoint).
+//! Each field is slot + 1 in one byte; 0 means none.
+
+use crate::bounded::CapSlot;
+
+const CONSOLE_SHIFT: u32 = 0;
+const LOG_SHIFT: u32 = 8;
+
+fn field(arg: usize, shift: u32) -> Option<CapSlot> {
+    let v = (arg >> shift) & 0xFF;
+    CapSlot::new(u8::try_from(v.checked_sub(1)?).ok()?)
+}
+
+fn put(slot: Option<CapSlot>, shift: u32) -> usize {
+    slot.map_or(0, |s| (s.value() as usize + 1) << shift)
+}
+
+pub fn encode(console: Option<CapSlot>, log: Option<CapSlot>) -> usize {
+    put(console, CONSOLE_SHIFT) | put(log, LOG_SHIFT)
+}
+
+pub fn console(arg: usize) -> Option<CapSlot> {
+    field(arg, CONSOLE_SHIFT)
+}
+
+pub fn log(arg: usize) -> Option<CapSlot> {
+    field(arg, LOG_SHIFT)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn slots_round_trip_and_zero_is_none() {
+        let (c, l) = (CapSlot::new(0), CapSlot::new(63));
+        let arg = encode(c, l);
+        assert_eq!((console(arg), log(arg)), (c, l));
+        assert_eq!((console(0), log(0)), (None, None));
+        assert_eq!(log(encode(c, None)), None);
+    }
+}