[PATCH] console: make input work again for pv-shim

[Jan Beulich <jbeulich@xxxxxxxx>]

From: Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>

The use of rcu_lock_domain_by_id() right in switch_serial_input() makes
assumptions about domain IDs which don't hold when in shim mode: The
sole (initial) domain there has a non-zero ID. Obtain the real domain ID
in that case (generalized as get_initial_domain_id() returns zero when
not in shim mode).

Note that console_input_domain() isn't altered, for not being used when
in shim mode (or more generally on x86).

Fixes: c2581c58bec9 ("xen/console: skip switching serial input to non existing 
domains")
Signed-off-by: Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -468,7 +468,7 @@ static void cf_check dump_console_ring_k
 #define switch_code (opt_conswitch[0]-'a'+1)
 /*
  * console_rx=0 => input to xen
- * console_rx=1 => input to dom0
+ * console_rx=1 => input to dom0 (or the sole shim domain)
  * console_rx=N => input to dom(N-1)
  */
 static unsigned int __read_mostly console_rx = 0;
@@ -493,6 +493,7 @@ static void switch_serial_input(void)
      */
     for ( ; ; )
     {
+        domid_t domid;
         struct domain *d;
 
         if ( next_rx++ >= max_console_rx )
@@ -502,12 +503,18 @@ static void switch_serial_input(void)
             break;
         }
 
-        d = rcu_lock_domain_by_id(next_rx - 1);
+#ifdef CONFIG_PV_SHIM
+        if ( next_rx == 1 )
+            domid = get_initial_domain_id();
+        else
+#endif
+            domid = next_rx - 1;
+        d = rcu_lock_domain_by_id(domid);
         if ( d )
         {
             rcu_unlock_domain(d);
             console_rx = next_rx;
-            printk("*** Serial input to DOM%u", next_rx - 1);
+            printk("*** Serial input to DOM%u", domid);
             break;
         }
     }