xen/common: add keyhandler to show Xen command line

Currently there's no way to print Xen command line on the emergency
console for debugging purposes when 'xl' is unavailable.

Add new print_cmdline() function to print both built-in and run-time
command lines on the console.

To allow built-in command printout, drop __initconst in
'opt_builtin_cmdline' declaration.

Add new keyhander '?' to show command line printout.

Signed-off-by: Denis Mukhin <dmukhin@ford.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/common/kernel.c

@@ -35,7 +35,7 @@ boolean_param("dit", opt_dit);
 #endif
 
 static xen_commandline_t __ro_after_init saved_cmdline;
-static const char __initconst opt_builtin_cmdline[] = CONFIG_CMDLINE;
+static const char opt_builtin_cmdline[] = CONFIG_CMDLINE;
 char __ro_after_init xen_cap_info[128];
 
 static int assign_integer_param(const struct kernel_param *param, uint64_t val)
@@ -758,6 +758,24 @@ long do_xen_version(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
     return -ENOSYS;
 }
 
+void print_cmdline(void)
+{
+    const char *cmdline;
+
+    if ( opt_builtin_cmdline[0] )
+        printk("Built-in command line: %s\n", opt_builtin_cmdline);
+
+    if ( IS_ENABLED(CONFIG_CMDLINE_OVERRIDE) )
+        cmdline = "<ignored> (CONFIG_CMDLINE_OVERRIDE=y)";
+    else if ( saved_cmdline[0] )
+        cmdline = saved_cmdline;
+    else
+        cmdline = NULL;
+
+    if ( cmdline )
+        printk("Command line: %s\n", cmdline);
+}
+
 /*
  * Local variables:
  * mode: C

xen/common/keyhandler.c

@@ -28,7 +28,7 @@ static unsigned char keypress_key;
 static bool alt_key_handling;
 
 static keyhandler_fn_t cf_check show_handlers, cf_check dump_hwdom_registers,
-    cf_check dump_domains, cf_check read_clocks;
+    cf_check dump_domains, cf_check read_clocks, cf_check show_system_info;
 static irq_keyhandler_fn_t cf_check do_toggle_alt_key, cf_check dump_registers,
     cf_check reboot_machine, cf_check run_all_keyhandlers;
 
@@ -57,6 +57,7 @@ static struct keyhandler {
     IRQ_KEYHANDLER('R', reboot_machine, "reboot machine", 0),
         KEYHANDLER('t', read_clocks, "display multi-cpu clock info", 1),
         KEYHANDLER('0', dump_hwdom_registers, "dump Dom0 registers", 1),
+        KEYHANDLER('?', show_system_info, "show system information", false),
     IRQ_KEYHANDLER('*', run_all_keyhandlers, "print all diagnostics", 0),
 
 #ifdef CONFIG_PERF_COUNTERS
@@ -138,6 +139,13 @@ static void cf_check show_handlers(unsigned char key)
                    isprint(i) ? i : ' ', i, key_table[i].desc);
 }
 
+static void cf_check show_system_info(unsigned char key)
+{
+    printk("'%c' pressed -> showing system information\n", key);
+
+    print_cmdline();
+}
+
 static cpumask_t dump_execstate_mask;
 
 void cf_check dump_execstate(const struct cpu_user_regs *regs)

xen/include/xen/lib.h

@@ -48,6 +48,7 @@ int parse_signed_integer(const char *name, const char *s, const char *e,
 int cmdline_strcmp(const char *frag, const char *name);
 
 void print_version(void);
+void print_cmdline(void);
 
 #ifdef CONFIG_DEBUG_TRACE
 extern void debugtrace_dump(void);