diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
index 8f41a06b93..3ab29b03ac 100644
--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -2370,9 +2370,14 @@ int ioapic_guest_write(unsigned long physbase, unsigned int reg, u32 val)
      * pirq and irq mapping. Where the GSI is greater than 256, we assume
      * that dom0 pirq == irq.
      */
-    pirq = (irq >= 256) ? irq : rte.vector;
-    if ( (pirq < 0) || (pirq >= dom0->nr_pirqs) )
-        return -EINVAL;
+    if ( !rte.mask )
+    {
+        pirq = (irq >= 256) ? irq : rte.vector;
+        if ( pirq >= dom0->nr_pirqs )
+            return -EINVAL;
+    }
+    else
+        pirq = -1;
     
     if ( desc->action )
     {
@@ -2407,12 +2412,14 @@ int ioapic_guest_write(unsigned long physbase, unsigned int reg, u32 val)
 
         printk(XENLOG_INFO "allocated vector %02x for irq %d\n", ret, irq);
     }
-    spin_lock(&dom0->event_lock);
-    ret = map_domain_pirq(dom0, pirq, irq,
-            MAP_PIRQ_TYPE_GSI, NULL);
-    spin_unlock(&dom0->event_lock);
-    if ( ret < 0 )
-        return ret;
+    if ( pirq >= 0 )
+    {
+        spin_lock(&dom0->event_lock);
+        ret = map_domain_pirq(dom0, pirq, irq, MAP_PIRQ_TYPE_GSI, NULL);
+        spin_unlock(&dom0->event_lock);
+        if ( ret < 0 )
+            return ret;
+    }
 
     spin_lock_irqsave(&ioapic_lock, flags);
     /* Set the correct irq-handling type. */
diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index 194d508143..77aded76c3 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -1897,7 +1897,7 @@ int map_domain_pirq(
     if ( !irq_access_permitted(current->domain, irq))
         return -EPERM;
 
-    if ( pirq < 0 || pirq >= d->nr_pirqs || irq < 0 || irq >= nr_irqs )
+    if ( pirq < 0 || pirq >= d->nr_pirqs || irq <= 0 || irq >= nr_irqs )
     {
         dprintk(XENLOG_G_ERR, "dom%d: invalid pirq %d or irq %d\n",
                 d->domain_id, pirq, irq);
@@ -1910,8 +1910,9 @@ int map_domain_pirq(
     if ( (old_irq > 0 && (old_irq != irq) ) ||
          (old_pirq && (old_pirq != pirq)) )
     {
-        dprintk(XENLOG_G_WARNING, "dom%d: pirq %d or irq %d already mapped\n",
-                d->domain_id, pirq, irq);
+        dprintk(XENLOG_G_WARNING,
+                "dom%d: pirq %d or irq %d already mapped (%d,%d)\n",
+                d->domain_id, pirq, irq, old_pirq, old_irq);
         return 0;
     }