doc: update 'enable_ivshmem'

Add notification (doorbell) usage for ivshmem devices.

Signed-off-by: Yonghua Huang <yonghua.huang@intel.com>
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/tutorials/enable_ivshmem.rst

@@ -66,6 +66,32 @@ enable it using the  :ref:`acrn_configuration_tool` with these steps:
 
 - Build the XML configuration, refer to :ref:`getting-started-building`
 
+ivshmem notification mechanism
+******************************
+
+Notification (doorbell) of ivshmem device allows VMs with ivshmem
+devices enabled to notify (interrupt) each other following this flow:
+
+Notification Sender (VM):
+   VM triggers the notification to target VM by writing target Peer ID
+   (Equals to VM ID of target VM) and vector index to doorbell register of
+   ivshmem device, the layout of doorbell register is described in
+   :ref:`ivshmem-hld`.
+
+Hypervisor:
+   When doorbell register is programmed, hypervisor will search the
+   target VM by target Peer ID and inject MSI interrupt to the target VM.
+
+Notification Receiver (VM):
+   VM receives MSI interrupt and forward it to related application.
+
+ACRN supports up to 8 (MSI-X) interrupt vectors for ivshmem device.
+Guest VMs shall implement their own mechanism to forward MSI interrupts
+to applications.
+
+.. note:: Notification is supported only for HV-land ivshmem devices. (Future
+   support may include notification for DM-land ivshmem devices.)
+
 Inter-VM Communication Examples
 *******************************