diff --git a/tools/libacpi/acpi2_0.h b/tools/libacpi/acpi2_0.h
index 2619ba3..ada5131 100644
--- a/tools/libacpi/acpi2_0.h
+++ b/tools/libacpi/acpi2_0.h
@@ -322,6 +322,7 @@ struct acpi_20_waet {
 #define ACPI_IO_SAPIC                       0x06
 #define ACPI_PROCESSOR_LOCAL_SAPIC          0x07
 #define ACPI_PLATFORM_INTERRUPT_SOURCES     0x08
+#define ACPI_PROCESSOR_LOCAL_X2APIC         0x09
 
 /*
  * APIC Structure Definitions.
@@ -338,6 +339,15 @@ struct acpi_20_madt_lapic {
     uint32_t flags;
 };
 
+struct acpi_20_madt_x2apic {
+    uint8_t  type;
+    uint8_t  length;
+    uint16_t reserved;          /* reserved - must be zero */
+    uint32_t apic_id;           /* Processor x2APIC ID  */
+    uint32_t flags;
+    uint32_t acpi_processor_id; /* ACPI processor UID */
+};
+
 /*
  * Local APIC Flags.  All other bits are reserved and must be 0.
  */
diff --git a/tools/libacpi/build.c b/tools/libacpi/build.c
index f9881c9..4830339 100644
--- a/tools/libacpi/build.c
+++ b/tools/libacpi/build.c
@@ -78,9 +78,9 @@ static struct acpi_20_madt *construct_madt(struct acpi_ctxt *ctxt,
     struct acpi_20_madt           *madt;
     struct acpi_20_madt_intsrcovr *intsrcovr;
     struct acpi_20_madt_ioapic    *io_apic;
-    struct acpi_20_madt_lapic     *lapic;
     const struct hvm_info_table   *hvminfo = config->hvminfo;
     int i, sz;
+    void *end;
 
     if ( config->lapic_id == NULL )
         return NULL;
@@ -88,7 +88,14 @@ static struct acpi_20_madt *construct_madt(struct acpi_ctxt *ctxt,
     sz  = sizeof(struct acpi_20_madt);
     sz += sizeof(struct acpi_20_madt_intsrcovr) * 16;
     sz += sizeof(struct acpi_20_madt_ioapic);
-    sz += sizeof(struct acpi_20_madt_lapic) * hvminfo->nr_vcpus;
+
+    for ( i = 0; i < hvminfo->nr_vcpus; i++ )
+    {
+        if ( config->lapic_id(i) > 254)
+            sz += sizeof(struct acpi_20_madt_x2apic);
+        else
+            sz += sizeof(struct acpi_20_madt_lapic);
+    }
 
     madt = ctxt->mem_ops.alloc(ctxt, sz, 16);
     if (!madt) return NULL;
@@ -142,27 +149,46 @@ static struct acpi_20_madt *construct_madt(struct acpi_ctxt *ctxt,
         io_apic->ioapic_id   = config->ioapic_id;
         io_apic->ioapic_addr = config->ioapic_base_address;
 
-        lapic = (struct acpi_20_madt_lapic *)(io_apic + 1);
+        end = (struct acpi_20_madt_lapic *)(io_apic + 1);
     }
     else
-        lapic = (struct acpi_20_madt_lapic *)(madt + 1);
+        end = (struct acpi_20_madt_lapic *)(madt + 1);
 
     info->nr_cpus = hvminfo->nr_vcpus;
-    info->madt_lapic0_addr = ctxt->mem_ops.v2p(ctxt, lapic);
+    info->madt_lapic0_addr = ctxt->mem_ops.v2p(ctxt, end);
+
     for ( i = 0; i < hvminfo->nr_vcpus; i++ )
     {
-        memset(lapic, 0, sizeof(*lapic));
-        lapic->type    = ACPI_PROCESSOR_LOCAL_APIC;
-        lapic->length  = sizeof(*lapic);
-        /* Processor ID must match processor-object IDs in the DSDT. */
-        lapic->acpi_processor_id = i;
-        lapic->apic_id = config->lapic_id(i);
-        lapic->flags = (test_bit(i, hvminfo->vcpu_online)
-                        ? ACPI_LOCAL_APIC_ENABLED : 0);
-        lapic++;
+        unsigned int apic_id = config->lapic_id(i);
+
+        if ( apic_id < 255 ) {
+            struct acpi_20_madt_lapic *lapic = end;
+
+            memset(lapic, 0, sizeof(*lapic));
+            lapic->type    = ACPI_PROCESSOR_LOCAL_APIC;
+            lapic->length  = sizeof(*lapic);
+            /* Processor ID must match processor-object IDs in the DSDT. */
+            lapic->acpi_processor_id = i;
+            lapic->apic_id = apic_id;
+            lapic->flags = test_bit(i, hvminfo->vcpu_online)
+                            ? ACPI_LOCAL_APIC_ENABLED : 0;
+            end = ++lapic;
+        } else {
+            struct acpi_20_madt_x2apic *lapic = end;
+
+            memset(lapic, 0, sizeof(*lapic));
+            lapic->type    = ACPI_PROCESSOR_LOCAL_X2APIC;
+            lapic->length  = sizeof(*lapic);
+            /* Processor ID must match processor-object IDs in the DSDT. */
+            lapic->acpi_processor_id = i;
+            lapic->apic_id = apic_id;
+            lapic->flags =  test_bit(i, hvminfo->vcpu_online)
+                            ? ACPI_LOCAL_APIC_ENABLED : 0;
+            end = ++lapic;
+        }
     }
 
-    madt->header.length = (unsigned char *)lapic - (unsigned char *)madt;
+    madt->header.length = (unsigned char *)end - (unsigned char *)madt;
     set_checksum(madt, offsetof(struct acpi_header, checksum),
                  madt->header.length);
     info->madt_csum_addr =
