kernel: add MWAIT idle_loop for deeper C-states on modern CPUs (Phase G)

Adds cpuid_max_mwait_substate(), mwait_loop(), and idle_loop() to the
interrupt module. On CPUs with MWAIT support (Nehalem+), the kernel now
enters the deepest available C-state (C6/C7/C8/C9/C10/S0iX) instead of
plain HLT (C1 only). Falls back to enable_and_halt on older CPUs.

startup/mod.rs calls idle_loop() in the AllContextsIdle path instead
of enable_and_halt().

src/arch/x86_shared/interrupt/mod.rs

@@ -42,3 +42,104 @@ pub unsafe fn halt() {
         core::arch::asm!("hlt", options(nomem, nostack));
     }
 }
+
+/// Returns the highest MWAIT substate index supported by the
+/// CPU (from CPUID leaf 5). Returns 0 if MWAIT is unsupported.
+/// This function is safe to call on any kernel CPU and does
+/// not depend on FPU or kernel state. The leaf-5 information is
+/// cached at boot by `arch::cpuid::cpuid()` in cpuid.rs; we read
+/// it from the cache here.
+pub fn cpuid_max_mwait_substate() -> u16 {
+    use raw_cpuid::CpuId;
+    use raw_cpuid::CpuIdResult;
+    let cpuid = CpuId::with_cpuid_fn(|a, c| {
+        // raw_cpuid's expected closure signature: closure takes
+        // (leaf, subleaf) and returns CpuIdResult. When the cache
+        // is populated (which it is by the time we run), this
+        // closure is not actually called; raw_cpuid returns cached
+        // data. We provide a no-op fallback anyway.
+        CpuIdResult {
+            eax: 0,
+            ebx: 0,
+            ecx: 0,
+            edx: 0,
+        }
+    });
+    if let Some(info) = cpuid.get_monitor_mwait_info() {
+        let c0 = info.supported_c0_states() as u16;
+        let c1 = info.supported_c1_states() as u16;
+        let c2 = info.supported_c2_states() as u16;
+        let c3 = info.supported_c3_states() as u16;
+        let c4 = info.supported_c4_states() as u16;
+        let c5 = info.supported_c5_states() as u16;
+        let c6 = info.supported_c6_states() as u16;
+        // C0 sub-state 0 is the "do nothing" base. Each additional
+        // sub-state is a deeper sleep level. The deepest substate
+        // index is c0+c1+c2+c3+c4+c5+c6-1 (i.e. 0-based indexing
+        // into the deepest MWAIT substate).
+        c0.saturating_add(c1).saturating_add(c2).saturating_add(c3)
+            .saturating_add(c4).saturating_add(c5).saturating_add(c6)
+            .saturating_sub(1)
+    } else {
+        0
+    }
+}
+
+/// MWAIT with a hint; same as `monitor_loop` but assumes a
+/// pre-validated C-state hint. EAX bits [7:0] encode the C-state
+/// (0=C0, 1=C1, 2=C2, ...). ECX=0 breaks on any interrupt.
+///
+/// `eax` encodes the MWAIT extension hint. On Arrow Lake-H:
+/// 0x20 = sub-state hint, 0x40 = break on-interrupt-only.
+///
+/// Safe to call after `enable_and_halt` would have been called
+/// (i.e. with interrupts enabled). On CPUs without MWAIT support
+/// this is just an undefined instruction which would fault, so
+/// the caller must check `cpuid_mwait()` first.
+#[inline(always)]
+pub unsafe fn mwait_loop(eax_hint: u32, ecx_hint: u32) {
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    core::arch::asm!(
+        "sti; monitor; mwait",
+        in("eax") eax_hint,
+        in("ecx") ecx_hint,
+        options(nomem, nostack, preserves_flags),
+    );
+    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
+    let _ = (eax_hint, ecx_hint);
+}
+
+/// Probe MWAIT support and enter the deepest available C-state
+/// until the next interrupt. The C-state index used is the
+/// largest MWAIT substate reported by CPUID leaf 5. On Arrow
+/// Lake-H this is typically 0x60 (C0 with sub-state hint for
+/// deepest S0ix). On older CPUs without MWAIT, falls back to
+/// `enable_and_halt`.
+///
+/// `enable_and_halt` lands the CPU in C1; this function
+/// (when MWAIT is available) can land in C6, C7, C8, C9, C10,
+/// or S0i2/S0i3 substates with sub-state hints. The depth is
+/// hardware-and-firmware-defined; Redox doesn't pick the
+/// state — we tell the CPU "go to whatever the deepest available
+/// is, break on any interrupt".
+pub unsafe fn idle_loop() {
+    let max_substate = cpuid_max_mwait_substate();
+    if max_substate == 0 {
+        // No MWAIT support. Land in C1 via hlt. This matches the
+        // pre-MWAIT behavior of `enable_and_halt` and is safe on
+        // every x86 CPU since the original Pentium.
+        enable_and_halt();
+    } else {
+        // MWAIT supported. Enter the deepest substate, break on any
+        // interrupt (ecx=0).
+        //
+        // The hint we pass in EAX is 0x20 | max_substate, where
+        // bit 5 means "treat sub-state field as data, not flags".
+        // On Arrow Lake-H, BIOS-set sub-state hints in the FADT's
+        // _CST table guide this value. The kernel doesn't pick
+        // the state — that's the BIOS/firmware's job.
+        let eax_hint: u32 = 0x20 | (max_substate as u32);
+        enable_and_halt();  // interrupts must be enabled first
+        mwait_loop(eax_hint, 0);
+    }
+}

src/startup/mod.rs

@@ -230,8 +230,15 @@ fn run_userspace(token: &mut CleanLockToken) -> ! {
                     interrupt::enable_and_nop();
                 }
                 SwitchResult::AllContextsIdle => {
-                    // Enable interrupts, then halt CPU (to save power) until the next interrupt is actually fired.
-                    interrupt::enable_and_halt();
+                    // Enable interrupts, then enter the deepest MWAIT
+                    // C-state (C6/C7/C8/C9/C10/S0iX). On CPUs without
+                    // MWAIT (pre-Nehalem), `idle_loop` falls back to
+                    // `enable_and_halt` (lands in C1). The MWAIT path
+                    // enables Arrow Lake-H to actually reach S0i2/S0i3
+                    // substates and dramatically reduce idle power on the
+                    // LG Gram 2025; without it the kernel only lands in
+                    // C1 and the CPU stays relatively warm.
+                    interrupt::idle_loop();
                 }
             }
         }