diff --git a/docs/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5
index 56b1117..1ba026b 100644
--- a/docs/man/xl.cfg.pod.5
+++ b/docs/man/xl.cfg.pod.5
@@ -1484,10 +1484,21 @@ This set incorporates use of hypercalls for remote TLB flushing.
 This enlightenment may improve performance of Windows guests running
 on hosts with higher levels of (physical) CPU contention.
 
+=item B<apic_assist>
+
+This set incorporates use of the APIC assist page to avoid EOI of
+the local APIC.
+This enlightenment may improve performance of Windows guests,
+particularly those running PV drivers that make use of per-vcpu
+event channel upcall vectors.
+Note that this enlightenment will have no effect if the guest is
+using APICv posted interrupts.
+
 =item B<defaults>
 
 This is a special value that enables the default set of groups, which
-is currently the B<base>, B<freq> and B<time_ref_count> groups.
+is currently the B<base>, B<freq>, B<time_ref_count> and B<apic_assist>
+groups.
 
 =item B<all>
 
diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
index b825b98..09d3bca 100644
--- a/tools/libxl/libxl_dom.c
+++ b/tools/libxl/libxl_dom.c
@@ -211,6 +211,7 @@ static int hvm_set_viridian_features(libxl__gc *gc, uint32_t domid,
         libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_BASE);
         libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_FREQ);
         libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_TIME_REF_COUNT);
+        libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_APIC_ASSIST);
     }
 
     libxl_for_each_set_bit(v, info->u.hvm.viridian_enable) {
@@ -253,6 +254,9 @@ static int hvm_set_viridian_features(libxl__gc *gc, uint32_t domid,
     if (libxl_bitmap_test(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_HCALL_REMOTE_TLB_FLUSH))
         mask |= HVMPV_hcall_remote_tlb_flush;
 
+    if (libxl_bitmap_test(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_APIC_ASSIST))
+        mask |= HVMPV_apic_assist;
+
     if (mask != 0 &&
         xc_hvm_param_set(CTX->xch,
                          domid,
diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
index 632c009..e3be957 100644
--- a/tools/libxl/libxl_types.idl
+++ b/tools/libxl/libxl_types.idl
@@ -221,6 +221,7 @@ libxl_viridian_enlightenment = Enumeration("viridian_enlightenment", [
     (2, "time_ref_count"),
     (3, "reference_tsc"),
     (4, "hcall_remote_tlb_flush"),
+    (5, "apic_assist"),
     ])
 
 libxl_hdtype = Enumeration("hdtype", [
diff --git a/xen/arch/x86/hvm/viridian.c b/xen/arch/x86/hvm/viridian.c
index e990163..73ac29d 100644
--- a/xen/arch/x86/hvm/viridian.c
+++ b/xen/arch/x86/hvm/viridian.c
@@ -227,11 +227,6 @@ static void initialize_apic_assist(struct vcpu *v)
     void *va;
 
     /*
-     * We don't yet make use of the APIC assist page but by setting
-     * the CPUID3A_MSR_APIC_ACCESS bit in CPUID leaf 40000003 we are duty
-     * bound to support the MSR. We therefore do just enough to keep windows
-     * happy.
-     *
      * See section 13.3.4.1 of the specification for details of this
      * enlightenment.
      */
@@ -256,6 +251,7 @@ static void initialize_apic_assist(struct vcpu *v)
 
     v->arch.hvm_vcpu.viridian.apic_assist.page = page;
     v->arch.hvm_vcpu.viridian.apic_assist.va = va;
+    v->arch.hvm_vcpu.viridian.apic_assist.vector = -1;
     return;
 
  fail:
@@ -263,6 +259,59 @@ static void initialize_apic_assist(struct vcpu *v)
              page ? page_to_mfn(page) : INVALID_MFN);
 }
 
+static uint32_t *get_apic_assist_word(struct vcpu *v)
+{
+    if ( !(viridian_feature_mask(v->domain) & HVMPV_apic_assist) )
+        return NULL;
+
+    return v->arch.hvm_vcpu.viridian.apic_assist.va;
+}
+
+void viridian_start_apic_assist(struct vcpu *v, int vector)
+{
+    uint32_t *va = get_apic_assist_word(v);
+
+    if ( !va )
+        return;
+
+    /*
+     * If there is already an assist pending then something has gone
+     * wrong and the VM will most likely hang so force a crash now
+     * to make the problem clear.
+     */
+    if (v->arch.hvm_vcpu.viridian.apic_assist.vector >= 0)
+        domain_crash(v->domain);
+
+    v->arch.hvm_vcpu.viridian.apic_assist.vector = vector;
+    *va = 1u;
+}
+
+bool_t viridian_complete_apic_assist(struct vcpu *v, int *vector)
+{
+    uint32_t *va = get_apic_assist_word(v);
+
+    if ( !va )
+        return 0;
+
+    if ( *va & 1 )
+        return 0; /* Interrupt not yet processed by the guest */
+
+    *vector = v->arch.hvm_vcpu.viridian.apic_assist.vector;
+    v->arch.hvm_vcpu.viridian.apic_assist.vector = -1;
+    return 1;
+}
+
+void viridian_abort_apic_assist(struct vcpu *v)
+{
+    uint32_t *va = get_apic_assist_word(v);
+
+    if ( !va )
+        return;
+
+    *va = 0;
+    v->arch.hvm_vcpu.viridian.apic_assist.vector = -1;
+}
+
 static void teardown_apic_assist(struct vcpu *v)
 {
     struct page_info *page = v->arch.hvm_vcpu.viridian.apic_assist.page;
diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c
index 01a8430..aac4263 100644
--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -38,6 +38,7 @@
 #include <asm/hvm/support.h>
 #include <asm/hvm/vmx/vmx.h>
 #include <asm/hvm/nestedhvm.h>
+#include <asm/hvm/viridian.h>
 #include <public/hvm/ioreq.h>
 #include <public/hvm/params.h>
 
@@ -95,6 +96,18 @@ static int vlapic_find_highest_vector(const void *bitmap)
     return (fls(word[word_offset*4]) - 1) + (word_offset * 32);
 }
 
+static int vlapic_find_lowest_vector(const void *bitmap)
+{
+    const uint32_t *word = bitmap;
+    unsigned int word_offset;
+
+    /* Work forwards through the bitmap (first 32-bit word in every four). */
+    for ( word_offset = 0; word_offset < NR_VECTORS / 32; word_offset++)
+        if ( word[word_offset * 4] )
+            return (ffs(word[word_offset * 4]) - 1) + (word_offset * 32);
+
+    return -1;
+}
 
 /*
  * IRR-specific bitmap update & search routines.
@@ -1157,7 +1170,7 @@ int vlapic_virtual_intr_delivery_enabled(void)
 int vlapic_has_pending_irq(struct vcpu *v)
 {
     struct vlapic *vlapic = vcpu_vlapic(v);
-    int irr, isr;
+    int irr, vector, isr;
 
     if ( !vlapic_enabled(vlapic) )
         return -1;
@@ -1170,10 +1183,27 @@ int vlapic_has_pending_irq(struct vcpu *v)
          !nestedhvm_vcpu_in_guestmode(v) )
         return irr;
 
+    /*
+     * If APIC assist was used then there may have been no EOI so
+     * we need to clear the requisite bit from the ISR here, before
+     * comparing with the IRR.
+     */
+    if ( viridian_complete_apic_assist(v, &vector) &&
+         vector != -1 )
+        vlapic_clear_vector(vector, &vlapic->regs->data[APIC_ISR]);
+
     isr = vlapic_find_highest_isr(vlapic);
     isr = (isr != -1) ? isr : 0;
     if ( (isr & 0xf0) >= (irr & 0xf0) )
+    {
+        /*
+         * There's already a higher priority vector pending so
+         * we need to abort any previous APIC assist to ensure there
+         * is an EOI.
+         */
+        viridian_abort_apic_assist(v);
         return -1;
+    }
 
     return irr;
 }
@@ -1181,13 +1211,29 @@ int vlapic_has_pending_irq(struct vcpu *v)
 int vlapic_ack_pending_irq(struct vcpu *v, int vector, bool_t force_ack)
 {
     struct vlapic *vlapic = vcpu_vlapic(v);
+    int isr;
 
-    if ( force_ack || !vlapic_virtual_intr_delivery_enabled() )
-    {
-        vlapic_set_vector(vector, &vlapic->regs->data[APIC_ISR]);
-        vlapic_clear_irr(vector, vlapic);
-    }
+    if ( !force_ack &&
+         vlapic_virtual_intr_delivery_enabled() )
+        return 1;
+
+    if ( vlapic_test_vector(vector, &vlapic->regs->data[APIC_TMR]) )
+        goto done;
+
+    isr = vlapic_find_lowest_vector(&vlapic->regs->data[APIC_ISR]);
+    if ( isr >= 0 && isr < vector )
+        goto done;
+
+    /*
+     * This vector is edge triggered and there are no lower priority
+     * vectors pending, so we can use APIC assist to avoid exiting
+     * for EOI.
+     */
+    viridian_start_apic_assist(v, vector);
 
+done:
+    vlapic_set_vector(vector, &vlapic->regs->data[APIC_ISR]);
+    vlapic_clear_irr(vector, vlapic);
     return 1;
 }
 
diff --git a/xen/include/asm-x86/hvm/viridian.h b/xen/include/asm-x86/hvm/viridian.h
index ee3a120..e0a523e 100644
--- a/xen/include/asm-x86/hvm/viridian.h
+++ b/xen/include/asm-x86/hvm/viridian.h
@@ -25,6 +25,7 @@ struct viridian_vcpu
         union viridian_apic_assist msr;
         struct page_info *page;
         void *va;
+        int vector;
     } apic_assist;
 };
 
@@ -123,6 +124,10 @@ void viridian_time_ref_count_thaw(struct domain *d);
 
 void viridian_vcpu_deinit(struct vcpu *v);
 
+void viridian_start_apic_assist(struct vcpu *v, int vector);
+bool_t viridian_complete_apic_assist(struct vcpu *v, int *vector);
+void viridian_abort_apic_assist(struct vcpu *v);
+
 #endif /* __ASM_X86_HVM_VIRIDIAN_H__ */
 
 /*
diff --git a/xen/include/public/hvm/params.h b/xen/include/public/hvm/params.h
index 73d4718..e69c72c 100644
--- a/xen/include/public/hvm/params.h
+++ b/xen/include/public/hvm/params.h
@@ -115,12 +115,17 @@
 #define _HVMPV_hcall_remote_tlb_flush 4
 #define HVMPV_hcall_remote_tlb_flush (1 << _HVMPV_hcall_remote_tlb_flush)
 
+/* Use APIC assist */
+#define _HVMPV_apic_assist 5
+#define HVMPV_apic_assist (1 << _HVMPV_apic_assist)
+
 #define HVMPV_feature_mask \
         (HVMPV_base_freq | \
          HVMPV_no_freq | \
          HVMPV_time_ref_count | \
          HVMPV_reference_tsc | \
-         HVMPV_hcall_remote_tlb_flush)
+         HVMPV_hcall_remote_tlb_flush | \
+         HVMPV_apic_assist)
 
 #endif
 
