xen/domain: Introduce arch_init_idle_domain()

The idle domain causes a large amount of complexity in domain_create() because
of x86's need to initialise d->arch.ctxt_switch in arch_domain_create().

In order to address this, introduce an optional hook to perform extra
initialisation of the idle domain.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/common/domain.c

@@ -686,6 +686,9 @@ struct domain *domain_create(domid_t domid,
 
     rangeset_domain_initialise(d);
 
+    if ( is_idle_domain(d) )
+        arch_init_idle_domain(d);
+
     /* DOMID_{XEN,IO,etc} (other than IDLE) are sufficiently constructed. */
     if ( is_system_domain(d) && !is_idle_domain(d) )
         return d;

xen/include/xen/sched.h

@@ -728,6 +728,11 @@ struct domain *domain_create(domid_t domid,
                              struct xen_domctl_createdomain *config,
                              unsigned int flags);
 
+#ifndef arch_init_idle_domain
+/* Optional, if there's any construction necessary for DOMID_IDLE */
+static inline void arch_init_idle_domain(struct domain *d) {}
+#endif
+
 /*
  * rcu_lock_domain_by_id() is more efficient than get_domain_by_id().
  * This is the preferred function if the returned domain reference