power: proper shutdown via UEFI ResetSystem (ACPI S5)

- uefi.rs: full EFI_RUNTIME_SERVICES layout with reset_system at index 10
  (spec order); EfiResetSystem type + RESET_* constants
- power.rs: stash the runtime-services table pre-EBS; poweroff() calls
  ResetSystem(EfiResetShutdown) — still callable after ExitBootServices
  because we never call SetVirtualAddressMap, so the pointers keep
  working with physical addresses; falls back to the hlt loop
- shell: 'poweroff' and 'exit' shut the machine down (QEMU exits),
  'halt' keeps the plain hlt loop

3 files changed, 81 insertions, 2 deletions

diff --git a/rsnix/src/lib.rs b/rsnix/src/lib.rs
index d2dacf0..8c69b35 100644
--- a/rsnix/src/lib.rs
+++ b/rsnix/src/lib.rs
@@ -7,6 +7,7 @@ pub mod gop;
 pub mod interrupts;
 pub mod memory;
 pub mod paging;
+pub mod power;
 pub mod ps2;
 pub mod serial;
 pub mod uefi;
diff --git a/rsnix/src/power.rs b/rsnix/src/power.rs
new file mode 100644
index 0000000..cb5d296
--- /dev/null
+++ b/rsnix/src/power.rs
@@ -0,0 +1,48 @@
+// ---------------------------------------------------------------------------
+// Power — halt loop and ACPI shutdown via the UEFI ResetSystem runtime
+// service.
+//
+// The firmware's runtime-services table is stashed before ExitBootServices
+// (`init`).  Because we never call SetVirtualAddressMap, the service
+// pointers keep working with physical (identity) addresses after EBS, so
+// poweroff() can ask the firmware to shut the machine down (ACPI S5) — on
+// QEMU this makes the VM exit, on real hardware the power drops.
+// ---------------------------------------------------------------------------
+
+use crate::uefi::{RuntimeServices, EFI_SUCCESS, RESET_SHUTDOWN};
+
+static mut RUNTIME_SERVICES: *mut RuntimeServices = core::ptr::null_mut();
+
+/// Stash the runtime-services table pointer (call in Phase 1, pre-EBS).
+pub fn init(rt: *mut RuntimeServices) {
+    unsafe {
+        RUNTIME_SERVICES = rt;
+    }
+}
+
+/// Halt the CPU forever (interrupts off, hlt loop).
+pub fn halt() -> ! {
+    loop {
+        unsafe {
+            core::arch::asm!("cli; hlt", options(nomem, nostack));
+        }
+    }
+}
+
+/// Shut the machine down through `ResetSystem(EfiResetShutdown)`.  Falls
+/// back to `halt()` if no table was saved or the call failed.
+pub fn poweroff() -> ! {
+    unsafe {
+        let rt = RUNTIME_SERVICES;
+        if rt.is_null() {
+            halt();
+        }
+        let reset = (*rt).reset_system;
+        if reset as usize == 0 {
+            halt();
+        }
+        (reset)(RESET_SHUTDOWN, EFI_SUCCESS, 0, core::ptr::null());
+        // ResetSystem only returns on failure.
+        halt()
+    }
+}
diff --git a/rsnix/src/uefi.rs b/rsnix/src/uefi.rs
index 619739e..b1b5e48 100644
--- a/rsnix/src/uefi.rs
+++ b/rsnix/src/uefi.rs
@@ -86,12 +86,42 @@ pub struct SimpleTextInput {
 }
 
 // ---------------------------------------------------------------------------
-// Runtime services (stub — we don't use them)
+// Runtime services
 // ---------------------------------------------------------------------------
+
+/// `EFI_RESET_SYSTEM` — ResetType, ResetStatus, DataSize, ResetData.
+/// ResetType: 0 cold, 1 warm, 2 shutdown, 3 platform-specific.
+pub type EfiResetSystem =
+    unsafe extern "efiapi" fn(u32, u64, usize, *const u16) -> Status;
+
+pub const RESET_COLD: u32 = 0;
+#[allow(dead_code)]
+pub const RESET_WARM: u32 = 1;
+pub const RESET_SHUTDOWN: u32 = 2;
+#[allow(dead_code)]
+pub const RESET_PLATFORM_SPECIFIC: u32 = 3;
+
+/// EFI_RUNTIME_SERVICES — function pointers in spec order after the header.
+/// We only use ResetSystem; the rest stay opaque.  The table remains
+/// callable after ExitBootServices because we never call SetVirtualAddressMap,
+/// so the pointers keep working with physical (identity) addresses.
 #[repr(C)]
 pub struct RuntimeServices {
     _hdr: TableHeader,
-    _reserved: [*mut c_void; 30],
+    _get_time: *mut c_void,                  // 0
+    _set_time: *mut c_void,                  // 1
+    _get_wakeup_time: *mut c_void,           // 2
+    _set_wakeup_time: *mut c_void,           // 3
+    _set_virtual_address_map: *mut c_void,   // 4
+    _convert_pointer: *mut c_void,           // 5
+    _get_variable: *mut c_void,              // 6
+    _get_next_variable_name: *mut c_void,    // 7
+    _set_variable: *mut c_void,              // 8
+    _get_next_high_monotonic_count: *mut c_void, // 9
+    pub reset_system: EfiResetSystem,        // 10
+    _update_capsule: *mut c_void,            // 11
+    _query_capsule_capabilities: *mut c_void, // 12
+    _query_variable_info: *mut c_void,       // 13
 }
 
 // ---------------------------------------------------------------------------