m1x/x86_64 + kern: Add panic() implementation

Signed-off-by: Ian Moffett <ian@mirocom.org>

src/m1x/arch/x86_64/kern/panic_thunk.S

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2026, Mirocom Laboratories
+ * Provided under the BSD-3 clause
+ */
+
+    .text
+    .globl panic
+    .extern __panic
+panic:
+    mov $1, %rax                /* Lock ourselves */
+    xchg %rax, __mp_sync        /* -> __mp_sync */
+    testq $1, %rax              /* Are we another processor? */
+    jnz panic_lockout           /* Yes, enter lockout */
+    lea stack_top(%rip), %rsp   /* Load a sane stack */
+    jmp __panic
+panic_lockout:
+    cli
+    hlt
+    jmp panic_lockout
+
+    .section .data
+__mp_sync: .quad 0
+
+    .align 8
+    .section .bss
+stack: .fill 4096, 1, 0
+stack_top:

src/m1x/include/kern/panic.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2026, Mirocom Laboratories
+ * Provided under the BSD-3 clause
+ */
+
+#ifndef _KERN_PANIC_H_
+#define _KERN_PANIC_H_ 1
+
+#include <sys/cdefs.h>
+#include <stdarg.h>
+
+/*
+ * Signal to the user that something went seriously wrong and
+ * cease all system operation.
+ */
+__no_return void panic(const char *fmt, ...);
+
+/*
+ * Central panic handler
+ *
+ * XXX: This function should not be called directly! Use the
+ *      panic() function instead.
+ */
+__no_return void __panic(const char *fmt, ...);
+
+#endif  /* !_KERN_PANIC_H_ */

src/m1x/kern/kern_init.c

@@ -4,6 +4,7 @@
  */
 
 #include <dev/cons/cons.h>
+#include <kern/panic.h>
 #include <lib/printf.h>
 #include <hal/cpu.h>
 
@@ -35,5 +36,6 @@ kmain(void)
     /* Initialize the BSP */
     hal_cpu_init();
 
-    for (;;);
+    panic("end of kernel\n");
+    __builtin_unreachable();
 }

src/m1x/kern/kern_panic.c

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2026, Mirocom Laboratories
+ * Provided under the BSD-3 clause
+ */
+
+#include <kern/panic.h>
+#include <lib/printf.h>
+#include <hal/cpu.h>
+
+/*
+ * Even though the stack is guaranteed to be sane upon entry,
+ * it would still be wise to minimize its usage and therefore
+ * rely on globals.
+ */
+static char panic_buf[128];
+static va_list panic_ap;
+
+__no_return void
+__panic(const char *fmt, ...)
+{
+    va_start(panic_ap, fmt);
+    vsnprintf(panic_buf, sizeof(panic_buf), fmt, panic_ap);
+    printf("\033[31;40mpanic\033[0m: %s", panic_buf);
+
+    for (;;) {
+        hal_cpu_halt();
+    }
+}