fix: coretempd MSR probe for QEMU default machine type

On QEMU's default i440FX machine type, rdmsr on unsupported MSRs
(0x19c IA32_THERM_STATUS, 0x1a2 IA32_TEMPERATURE_TARGET) causes a
kernel #GP that kills the process. Same pattern as cpufreqd: spawn a
child with --probe-msr to test readability before the main loop. If
probe fails, disable all MSR reads and report all CPUs as Unknown.

local/recipes/system/coretempd/source/src/main.rs

@@ -1,5 +1,7 @@
 use std::env;
 use std::fs;
+use std::process::Command;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::thread;
 use std::time::Duration;
 
@@ -17,6 +19,8 @@ const IA32_THERM_STATUS: u32 = 0x19c;
 const IA32_TEMPERATURE_TARGET: u32 = 0x1a2;
 const AMD_TCTL: u32 = 0xc0010293;
 
+static MSR_AVAILABLE: AtomicBool = AtomicBool::new(true);
+
 #[derive(Clone, Copy, Debug, PartialEq)]
 enum Vendor {
     Intel,
@@ -25,12 +29,29 @@ enum Vendor {
 }
 
 fn read_msr(cpu: u32, msr: u32) -> Option<u64> {
+    if !MSR_AVAILABLE.load(Ordering::Relaxed) {
+        return None;
+    }
     let path = format!("/scheme/sys/msr/{}/{:x}", cpu, msr);
     fs::read_to_string(&path)
         .ok()
         .and_then(|s| u64::from_str_radix(s.trim(), 16).ok())
 }
 
+fn probe_msr_available() -> bool {
+    let exe = match env::current_exe() {
+        Ok(p) => p,
+        Err(_) => return false,
+    };
+    match Command::new(exe)
+        .arg("--probe-msr")
+        .status()
+    {
+        Ok(s) => s.success(),
+        Err(_) => false,
+    }
+}
+
 fn detect_vendor(cpu: u32) -> Vendor {
     if read_msr(cpu, IA32_THERM_STATUS).is_some() {
         Vendor::Intel
@@ -275,6 +296,15 @@ fn notify_scheme_ready(
 fn run_daemon() -> Result<(), String> {
     let notify_fd = init_notify_fd();
 
+    // Probe MSR availability before attempting any reads.
+    // On QEMU's default machine type, rdmsr on unsupported MSRs (0x19c, 0x1a2)
+    // causes a kernel #GP that kills the process. By spawning a child to test
+    // the read first, we detect this safely and degrade to Unknown vendor.
+    if !probe_msr_available() {
+        MSR_AVAILABLE.store(false, Ordering::Relaxed);
+        eprintln!("[INFO] coretempd: MSR reads unavailable — reporting all CPUs as Unknown");
+    }
+
     let cpus = detect_cpus();
     eprintln!("[INFO] coretempd: detected {} CPU(s)", cpus.len());
 
@@ -345,6 +375,13 @@ fn run_daemon() -> Result<(), String> {
 }
 
 fn main() {
+    // Probe mode: child process tests MSR read and exits with status.
+    let args: Vec<String> = env::args().collect();
+    if args.len() >= 2 && args[1] == "--probe-msr" {
+        let probe_ok = read_msr(0, IA32_THERM_STATUS).is_some();
+        std::process::exit(if probe_ok { 0 } else { 1 });
+    }
+
     if let Err(e) = run_daemon() {
         eprintln!("[ERROR] coretempd: {e}");
         std::process::exit(1);