xen/console: skip switching serial input to non existing domains

At the moment, we direct serial input to hardware domain by default.
This does not make any sense when running in true dom0less mode, since
such domain does not exist. As a result, users wishing to write to
an emulated UART of a domU are always forced to execute CTRL-AAA first.
The same issue is when rotating among serial inputs, where we always
have to go through hardware domain case. This problem can be elaborated
further to all the domains that no longer exist.

Modify switch_serial_input() so that we skip switching serial input to
non existing domains. Take the opportunity to define and make use of
macro max_console_rx to make it clear what 'max_init_domid + 1' means
in the console code context. Also, modify call to printk() to use correct
format specifier for unsigned int.

For now, to minimize the required changes and to match the current
behavior with hwdom, the default input goes to the first real domain.
The choice is more or less arbitrary since dom0less domUs are supposedly
equal. This will be handled in the future by adding support in boot time
configuration for marking a specific domain preferred in terms of
directing serial input to.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/drivers/char/console.c

@@ -473,6 +473,8 @@ static void cf_check dump_console_ring_key(unsigned char key)
  */
 static unsigned int __read_mostly console_rx = 0;
 
+#define max_console_rx (max_init_domid + 1)
+
 /* Make sure to rcu_unlock_domain after use */
 struct domain *console_input_domain(void)
 {
@@ -483,15 +485,31 @@ struct domain *console_input_domain(void)
 
 static void switch_serial_input(void)
 {
-    if ( console_rx == max_init_domid + 1 )
+    unsigned int next_rx = console_rx;
+
+    /*
+     * Rotate among Xen, dom0 and boot-time created domUs while skipping
+     * switching serial input to non existing domains.
+     */
+    for ( ; ; )
     {
-        console_rx = 0;
-        printk("*** Serial input to Xen");
-    }
-    else
-    {
-        console_rx++;
-        printk("*** Serial input to DOM%d", console_rx - 1);
+        struct domain *d;
+
+        if ( next_rx++ >= max_console_rx )
+        {
+            console_rx = 0;
+            printk("*** Serial input to Xen");
+            break;
+        }
+
+        d = rcu_lock_domain_by_id(next_rx - 1);
+        if ( d )
+        {
+            rcu_unlock_domain(d);
+            console_rx = next_rx;
+            printk("*** Serial input to DOM%u", next_rx - 1);
+            break;
+        }
     }
 
     if ( switch_code )
@@ -1089,7 +1107,7 @@ void __init console_endboot(void)
      * a useful 'how to switch' message.
      */
     if ( opt_conswitch[1] == 'x' )
-        console_rx = max_init_domid + 1;
+        console_rx = max_console_rx;
 
     register_keyhandler('w', dump_console_ring_key,
                         "synchronously dump console ring buffer (dmesg)", 0);