diff --git a/docs/misc/viommu.txt b/docs/misc/viommu.txt
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+Xen virtual IOMMU
+
+Motivation
+==========
+*) Enable more than 255 vcpu support
+HPC cloud service requires VM provides high performance parallel
+computing and we hope to create a huge VM with >255 vcpu on one machine
+to meet such requirement. Pin each vcpu to separate pcpus.
+
+To support >255 vcpus, X2APIC mode in guest is necessary because legacy
+APIC(XAPIC) just supports 8-bit APIC ID and it only can support 255
+vcpus at most. X2APIC mode supports 32-bit APIC ID and it requires
+interrupt mapping function of vIOMMU.
+
+The reason for this is that there is no modification to existing PCI MSI
+and IOAPIC with the introduction of X2APIC. PCI MSI/IOAPIC can only send
+interrupt message containing 8-bit APIC ID, which cannot address >255
+cpus. Interrupt remapping supports 32-bit APIC ID and so it's necessary
+to enable >255 cpus with x2apic mode.
+
+
+vIOMMU Architecture
+===================
+vIOMMU device model is inside Xen hypervisor for following factors
+    1) Avoid round trips between Qemu and Xen hypervisor
+    2) Ease of integration with the rest of hypervisor
+    3) HVMlite/PVH doesn't use Qemu
+
+* Interrupt remapping overview.
+Interrupts from virtual devices and physical devices are delivered
+to vLAPIC from vIOAPIC and vMSI. vIOMMU needs to remap interrupt during
+this procedure.
+
++---------------------------------------------------+
+|Qemu                       |VM                     |
+|                           | +----------------+    |
+|                           | |  Device driver |    |
+|                           | +--------+-------+    |
+|                           |          ^            |
+|       +----------------+  | +--------+-------+    |
+|       | Virtual device |  | |  IRQ subsystem |    |
+|       +-------+--------+  | +--------+-------+    |
+|               |           |          ^            |
+|               |           |          |            |
++---------------------------+-----------------------+
+|hyperviosr     |                      | VIRQ       |
+|               |            +---------+--------+   |
+|               |            |      vLAPIC      |   |
+|               |VIRQ        +---------+--------+   |
+|               |                      ^            |
+|               |                      |            |
+|               |            +---------+--------+   |
+|               |            |      vIOMMU      |   |
+|               |            +---------+--------+   |
+|               |                      ^            |
+|               |                      |            |
+|               |            +---------+--------+   |
+|               |            |   vIOAPIC/vMSI   |   |
+|               |            +----+----+--------+   |
+|               |                 ^    ^            |
+|               +-----------------+    |            |
+|                                      |            |
++---------------------------------------------------+
+HW                                     |IRQ
+                                +-------------------+
+                                |   PCI Device      |
+                                +-------------------+
+
+
+vIOMMU hypercall
+================
+Introduce new domctl hypercall "xen_domctl_viommu_op" to create/destroy
+vIOMMU and query vIOMMU capabilities that device model can support.
+
+* vIOMMU hypercall parameter structure
+struct xen_domctl_viommu_op {
+    uint32_t cmd;
+#define XEN_DOMCTL_create_viommu          0
+#define XEN_DOMCTL_destroy_viommu         1
+#define XEN_DOMCTL_query_viommu_caps      2
+    union {
+        struct {
+            /* IN - vIOMMU type */
+            uint64_t viommu_type;
+            /* IN - MMIO base address of vIOMMU. */
+            uint64_t base_address;
+            /* IN - Length of MMIO region */
+            uint64_t length;
+            /* IN - Capabilities with which we want to create */
+            uint64_t capabilities;
+            /* OUT - vIOMMU identity */
+            uint32_t viommu_id;
+        } create_viommu;
+
+        struct {
+            /* IN - vIOMMU identity */
+            uint32_t viommu_id;
+        } destroy_viommu;
+
+        struct {
+            /* IN - vIOMMU type */
+            uint64_t viommu_type;
+            /* OUT - vIOMMU Capabilities */
+            uint64_t caps;
+        } query_caps;
+    } u;
+};
+
+- XEN_DOMCTL_query_viommu_caps
+    Query capabilities of vIOMMU device model. vIOMMU_type specifies
+which vendor vIOMMU device model(E,G Intel VTD) is targeted and hypervisor
+returns capability bits(E,G interrupt remapping bit).
+
+- XEN_DOMCTL_create_viommu
+    Create vIOMMU device with vIOMMU_type, capabilities, MMIO
+base address and length. Hypervisor returns viommu_id. Capabilities should
+be in range of value returned by query_viommu_caps hypercall.
+
+- XEN_DOMCTL_destroy_viommu
+    Destroy vIOMMU in Xen hypervisor with viommu_id as parameters.
+
+xl vIOMMU configuration
+=======================
+viommu="type=vtd,intremap=1,x2apic=1"
+
+"type" - Specify vIOMMU device model type. Currently only supports Intel vtd
+device model.
+"intremap" - Enable vIOMMU interrupt remapping function.
+"x2apic" - Support x2apic mode with interrupt remapping function.
