x86/vpt: fully init timers before putting onto list

With pt_vcpu_lock() no longer acquiring the pt_migrate lock, parties
iterating the list and acting on the timers of the list entries will no
longer be kept from entering their loops by create_periodic_time()'s
holding of that lock. Therefore at least init_timer() needs calling
ahead of list insertion, but keep this and set_timer() together.

Fixes: 8113b02f0b ("x86/vpt: do not take pt_migrate rwlock in some cases")
Reported-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
master commit: 6d622f3a96
master date: 2021-06-15 15:14:20 +0200

xen/arch/x86/hvm/vpt.c

@@ -554,14 +554,14 @@ void create_periodic_time(
     pt->cb = cb;
     pt->priv = data;
 
+    init_timer(&pt->timer, pt_timer_fn, pt, v->processor);
+    set_timer(&pt->timer, pt->scheduled);
+
     pt_vcpu_lock(v);
     pt->on_list = 1;
     list_add(&pt->list, &v->arch.hvm.tm_list);
     pt_vcpu_unlock(v);
 
-    init_timer(&pt->timer, pt_timer_fn, pt, v->processor);
-    set_timer(&pt->timer, pt->scheduled);
-
     write_unlock(&v->domain->arch.hvm.pl_time->pt_migrate);
 }