adjust a few RCU domain locking calls

x86's do_physdev_op() had a case where the locking was entirely
superfluous. Its physdev_map_pirq() further had a case where the lock
was being obtained too early, needlessly complicating early exit paths.

Grant table code had two open coded instances of
rcu_lock_target_domain_by_id(), and a third code section could be
consolidated by using the newly introduced helper function.

The memory hypercall code had two more instances of open coding
rcu_lock_target_domain_by_id(), but note that here this is not just
cleanup, but also fixes an error return path in memory_exchange() to
actually return an error.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>

1 files changed, 9 insertions, 14 deletions

diff --git a/xen/arch/x86/physdev.c b/xen/arch/x86/physdev.c
index 8e7bc24b62..ce55ee1f7a 100644
--- a/xen/arch/x86/physdev.c
+++ b/xen/arch/x86/physdev.c
@@ -90,14 +90,10 @@ static int physdev_hvm_map_pirq(
 int physdev_map_pirq(domid_t domid, int type, int *index, int *pirq_p,
                      struct msi_info *msi)
 {
-    struct domain *d;
+    struct domain *d = current->domain;
     int pirq, irq, ret = 0;
     void *map_data = NULL;
 
-    ret = rcu_lock_target_domain_by_id(domid, &d);
-    if ( ret )
-        return ret;
-
     if ( domid == DOMID_SELF && is_hvm_domain(d) )
     {
         /*
@@ -105,14 +101,15 @@ int physdev_map_pirq(domid_t domid, int type, int *index, int *pirq_p,
          * calls back into itself and deadlocks on hvm_domain.irq_lock.
          */
         if ( !is_hvm_pv_evtchn_domain(d) )
-        {
-            ret = -EINVAL;
-            goto free_domain;
-        }
-        ret = physdev_hvm_map_pirq(d, type, index, pirq_p);
-        goto free_domain;
+            return -EINVAL;
+
+        return physdev_hvm_map_pirq(d, type, index, pirq_p);
     }
 
+    ret = rcu_lock_target_domain_by_id(domid, &d);
+    if ( ret )
+        return ret;
+
     if ( !IS_PRIV_FOR(current->domain, d) )
     {
         ret = -EPERM;
@@ -696,13 +693,12 @@ ret_t do_physdev_op(int cmd, XEN_GUEST_HANDLE(void) arg)
     }
     case PHYSDEVOP_get_free_pirq: {
         struct physdev_get_free_pirq out;
-        struct domain *d;
+        struct domain *d = v->domain;
 
         ret = -EFAULT;
         if ( copy_from_guest(&out, arg, 1) != 0 )
             break;
 
-        d = rcu_lock_current_domain();
         spin_lock(&d->event_lock);
 
         ret = get_free_pirq(d, out.type);
@@ -717,7 +713,6 @@ ret_t do_physdev_op(int cmd, XEN_GUEST_HANDLE(void) arg)
         }
 
         spin_unlock(&d->event_lock);
-        rcu_unlock_domain(d);
 
         if ( ret >= 0 )
         {