diff --git a/xen/arch/x86/apic.c b/xen/arch/x86/apic.c
index 50f3d305e8..7e84daf32a 100644
--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -528,9 +528,9 @@ void setup_local_APIC(void)
     init_apic_ldr();
 
     /*
-     * Set Task Priority to reject any interrupts below FIRST_DYNAMIC_VECTOR.
+     * Set Task Priority to reject any interrupts below FIRST_IRQ_VECTOR.
      */
-    apic_write(APIC_TASKPRI, (FIRST_DYNAMIC_VECTOR & 0xF0) - 0x10);
+    apic_write(APIC_TASKPRI, (FIRST_IRQ_VECTOR & 0xF0) - 0x10);
 
     /*
      * After a crash, we no longer service the interrupts and a pending
diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
index 6238df494b..9913dfde1f 100644
--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -2388,7 +2388,9 @@ int ioapic_guest_write(unsigned long physbase, unsigned int reg, u32 val)
         return 0;
     }
 
-    if ( desc->arch.vector <= 0 || desc->arch.vector > LAST_DYNAMIC_VECTOR )
+    if ( desc->arch.vector <= 0 || desc->arch.vector > LAST_DYNAMIC_VECTOR ||
+         (desc->arch.vector >= FIRST_LEGACY_VECTOR &&
+          desc->arch.vector <= LAST_LEGACY_VECTOR) )
     {
         int vector = desc->arch.vector;
 
diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index 9adad3ff04..ae02cf1712 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -101,7 +101,7 @@ void unlock_vector_lock(void)
 
 static inline bool valid_irq_vector(unsigned int vector)
 {
-    return vector >= FIRST_DYNAMIC_VECTOR && vector <= LAST_HIPRIORITY_VECTOR;
+    return vector >= FIRST_IRQ_VECTOR && vector <= LAST_IRQ_VECTOR;
 }
 
 static void release_old_vec(struct irq_desc *desc)
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 11385857fa..70c0b1c0a5 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -923,9 +923,9 @@ autogen_stubs: /* Automatically generated stubs. */
 
         /* Common interrupts, heading towards do_IRQ(). */
 #ifdef CONFIG_PV
-        .if vec >= FIRST_DYNAMIC_VECTOR && vec != HYPERCALL_VECTOR && vec != LEGACY_SYSCALL_VECTOR
+        .if vec >= FIRST_IRQ_VECTOR && vec != HYPERCALL_VECTOR && vec != LEGACY_SYSCALL_VECTOR
 #else
-        .if vec >= FIRST_DYNAMIC_VECTOR
+        .if vec >= FIRST_IRQ_VECTOR
 #endif
 
         ALIGN
diff --git a/xen/include/asm-x86/mach-default/irq_vectors.h b/xen/include/asm-x86/mach-default/irq_vectors.h
index 992e00c44b..6824a9e301 100644
--- a/xen/include/asm-x86/mach-default/irq_vectors.h
+++ b/xen/include/asm-x86/mach-default/irq_vectors.h
@@ -18,20 +18,23 @@
 /* IRQ0 (timer) is statically allocated but must be high priority. */
 #define IRQ0_VECTOR             0xf0
 
-/* Legacy PIC uses vectors 0xe0-0xef. */
-#define FIRST_LEGACY_VECTOR	0xe0
-#define LAST_LEGACY_VECTOR      0xef
+/* Legacy PIC uses vectors 0x20-0x2f. */
+#define FIRST_LEGACY_VECTOR     0x20
+#define LAST_LEGACY_VECTOR      (FIRST_LEGACY_VECTOR + 0xf)
 
 #define HYPERCALL_VECTOR	0x82
 #define LEGACY_SYSCALL_VECTOR   0x80
 
 /* Dynamically-allocated vectors available to any driver. */
-#define FIRST_DYNAMIC_VECTOR	0x20
-#define LAST_DYNAMIC_VECTOR	0xdf
+#define FIRST_DYNAMIC_VECTOR    (LAST_LEGACY_VECTOR + 1)
+#define LAST_DYNAMIC_VECTOR     0xef
 #define NR_DYNAMIC_VECTORS	(LAST_DYNAMIC_VECTOR - FIRST_DYNAMIC_VECTOR + 1)
 
 #define IRQ_MOVE_CLEANUP_VECTOR FIRST_DYNAMIC_VECTOR
 
 #define NR_VECTORS 256
 
+#define FIRST_IRQ_VECTOR        FIRST_LEGACY_VECTOR
+#define LAST_IRQ_VECTOR         LAST_HIPRIORITY_VECTOR
+
 #endif /* _ASM_IRQ_VECTORS_H */