diff -r 0268e7380953 xen/arch/x86/io_apic.c
--- a/xen/arch/x86/io_apic.c	Mon Sep 05 15:10:28 2011 +0100
+++ b/xen/arch/x86/io_apic.c	Thu Sep 08 14:17:31 2011 +0100
@@ -357,7 +357,7 @@ static void __eoi_IO_APIC_irq(unsigned i
         pin = entry->pin;
         if (pin == -1)
             break;
-        io_apic_eoi(entry->apic, vector);
+        io_apic_eoi_vector(entry->apic, vector);
         if (!entry->next)
             break;
         entry = irq_2_pin + entry->next;
@@ -397,18 +397,7 @@ static void clear_IO_APIC_pin(unsigned i
             entry.trigger = 1;
             __ioapic_write_entry(apic, pin, TRUE, entry);
         }
-        if (mp_ioapics[apic].mpc_apicver >= 0x20)
-            io_apic_eoi(apic, entry.vector);
-        else {
-            /*
-             * Mechanism by which we clear remoteIRR in this case is by
-             * changing the trigger mode to edge and back to level.
-             */
-            entry.trigger = 0;
-            __ioapic_write_entry(apic, pin, TRUE, entry);
-            entry.trigger = 1;
-            __ioapic_write_entry(apic, pin, TRUE, entry);
-        }
+        io_apic_eoi_pin(apic, pin);
     }
 
     /*
@@ -1750,7 +1739,7 @@ static void end_level_ioapic_irq (unsign
     {
         int ioapic;
         for (ioapic = 0; ioapic < nr_ioapics; ioapic++)
-            io_apic_eoi(ioapic, i);
+            io_apic_eoi_vector(ioapic, i);
     }
 
     v = apic_read(APIC_TMR + ((i & ~0x1f) >> 1));
@@ -2622,3 +2611,67 @@ void __init init_ioapic_mappings(void)
     printk(XENLOG_INFO "IRQ limits: %u GSI, %u MSI/MSI-X\n",
            nr_irqs_gsi, nr_irqs - nr_irqs_gsi);
 }
+
+/* EOI an IO-APIC entry given the vector it points to */
+void io_apic_eoi_vector(unsigned int apic, unsigned int vector)
+{
+    unsigned int flags;
+    
+    spin_lock_irqsave(&ioapic_lock, flags);
+
+    if ( ioapic_has_eoi_reg(apic) )
+        /* Prefer the use of the EOI register if available */
+        *(IO_APIC_BASE(apic)+16) = vector;
+    else
+    {
+        /* Else search through the IO-APIC entries to find the
+         * correct pin */
+        struct IO_APIC_route_entry entry;
+        unsigned int pin = 0;
+
+        do
+        {
+            entry = __ioapic_read_entry(apic, pin, TRUE);
+
+            if ( entry.vector == vector )
+            {
+                /* And if found, fake an EOI by flipping the
+                 * trigger mode to edge and back */
+                entry.trigger = 0;
+                __ioapic_write_entry(apic, pin, TRUE, entry);
+                entry.trigger = 1;
+                __ioapic_write_entry(apic, pin, TRUE, entry);
+                break;
+            }
+
+        } while ( pin++ < nr_ioapic_registers[apic] );
+    }
+
+    spin_unlock_irqrestore(&ioapic_lock, flags);
+}
+
+/* EOI an IO-APIC entry given its pin */
+void io_apic_eoi_pin(unsigned int apic, unsigned int pin)
+{
+    struct IO_APIC_route_entry entry;
+    unsigned int flags;
+
+    entry = __ioapic_read_entry(apic, pin, TRUE);
+
+    spin_lock_irqsave(&ioapic_lock, flags);
+
+    if ( ioapic_has_eoi_reg(apic) )
+        /* Prefer the use of the EOI register if available */
+        *(IO_APIC_BASE(apic)+16) = entry.vector;
+    else
+    {
+        /* Else fake an EOI by switching to edge triggered mode
+         * and back */
+        entry.trigger = 0;
+        __ioapic_write_entry(apic, pin, TRUE, entry);
+        entry.trigger = 1;
+        __ioapic_write_entry(apic, pin, TRUE, entry);
+    }
+
+    spin_unlock_irqrestore(&ioapic_lock, flags);
+}
diff -r 0268e7380953 xen/include/asm-x86/io_apic.h
--- a/xen/include/asm-x86/io_apic.h	Mon Sep 05 15:10:28 2011 +0100
+++ b/xen/include/asm-x86/io_apic.h	Thu Sep 08 14:17:31 2011 +0100
@@ -157,10 +157,9 @@ static inline void io_apic_write(unsigne
 	__io_apic_write(apic, reg, value);
 }
 
-static inline void io_apic_eoi(unsigned int apic, unsigned int vector)
-{
-	*(IO_APIC_BASE(apic)+16) = vector;
-}
+#define ioapic_has_eoi_reg(apic) (mp_ioapics[(apic)].mpc_apicver >= 0x20)
+void io_apic_eoi_vector(unsigned int apic, unsigned int vector);
+void io_apic_eoi_pin(unsigned int apic, unsigned int pin);
 
 /*
  * Re-write a value: to be used for read-modify-write