docs: acrn: update the documentation of setting the ACRN hypervisor

Add some descriptions of hybrid scenario of ACRN hypervisor, and
completed the configurations that we are using to build ACRN. This
configuration change for ACRN hypervisor is necessary when our Zephyr
application is using over than one CPU for it.

Signed-off-by: Enjia Mai <enjia.mai@intel.com>

boards/x86/acrn/doc/index.rst

@@ -5,6 +5,28 @@ Zephyr's is capable of running as a guest under the x86 ACRN
 hypervisor (see https://projectacrn.org/).  The process for getting
 this to work is somewhat involved, however.
 
+ACRN hypervisor supports a hybrid scenario where Zephyr runs in a so-
+called "pre-launched" mode. This means Zephyr will access the ACRN
+hypervisor directly without involving the SOS VM. This is the most
+practical user scenario in the real world because Zephyr's real-time
+and safety capability can be assured without influence from other
+VMs. The following figure from ACRN's official documentation shows
+how a hybrid scenario works:
+
+.. figure:: ACRN-Hybrid.png
+    :align: center
+    :alt: ACRN Hybrid User Scenario
+    :figclass: align-center
+    :width: 80%
+
+    ACRN Hybrid User Scenario
+
+In this tutorial, we will show you how to build a minimal running instance of Zephyr
+and ACRN hypervisor to demonstrate that it works successfully. To learn more about
+other features of ACRN, such as building and using the SOS VM or other guest VMs,
+please refer to the Getting Started Guide for ACRN:
+https://projectacrn.github.io/latest/tutorials/using_hybrid_mode_on_nuc.html
+
 Build your Zephyr App
 *********************
 
@@ -15,6 +37,10 @@ normally would, selecting an appropriate board:
 
         west build -b acrn_ehl_crb samples/hello_world
 
+In this tutorial, we will use the Intel Elkhart Lake Reference Board
+(`EHL`_ CRB) since it is one of the suggested platform for this
+type of scenario. Use ``acrn_ehl_crb`` as the target board parameter.
+
 Note the kconfig output in ``build/zephyr/.config``, you will need to
 reference that to configure ACRN later.
 
@@ -31,6 +57,9 @@ First you need the source code, clone from:
 
         git clone https://github.com/projectacrn/acrn-hypervisor
 
+We suggest that you use versions v2.5.1 or later of the ACRN hypervisor
+as they have better support for SMP in Zephyr.
+
 Like Zephyr, ACRN favors build-time configuration management instead
 of runtime probing or control.  Unlike Zephyr, ACRN has single large
 configuration files instead of small easily-merged configuration
@@ -102,10 +131,51 @@ many CPUs in the ``<cpu_affinity>`` tag.  For example:
 
     .. code-block:: xml
 
-        <cpu_affinity>
-            <pcpu_id>0</pcpu_id>
-            <pcpu_id>1</pcpu_id>
-        </cpu_affinity>
+        <vm id="0">
+            <vm_type>SAFETY_VM</vm_type>
+            <name>ACRN PRE-LAUNCHED VM0</name>
+            <guest_flags>
+                <guest_flag>0</guest_flag>
+            </guest_flags>
+            <cpu_affinity>
+                <pcpu_id>0</pcpu_id>
+                <pcpu_id>1</pcpu_id>
+            </cpu_affinity>
+            ...
+            <clos>
+                <vcpu_clos>0</vcpu_clos>
+                <vcpu_clos>0</vcpu_clos>
+            </clos>
+            ...
+        </vm>
+
+To use SMP, we have to change the pcpu_id of VM0 to 0 and 1.
+This configures ACRN to run Zephyr on CPU0 and CPU1. The ACRN hypervisor
+and Zephyr application will not boot successfully without this change.
+If you plan to run Zephyr with one CPU only, you can skip it.
+
+Since Zephyr is using CPU0 and CPU1, we also have to change
+VM1's configuration so it runs on CPU2 and CPU3. If your ACRN set up has
+additional VMs, you should change their configurations as well.
+
+    .. code-block:: xml
+
+        <vm id="1">
+            <vm_type>SOS_VM</vm_type>
+            <name>ACRN SOS VM</name>
+            <guest_flags>
+                <guest_flag>0</guest_flag>
+            </guest_flags>
+            <cpu_affinity>
+                <pcpu_id>2</pcpu_id>
+                <pcpu_id>3</pcpu_id>
+            </cpu_affinity>
+            <clos>
+                <vcpu_clos>0</vcpu_clos>
+                <vcpu_clos>0</vcpu_clos>
+            </clos>
+            ...
+        </vm>
 
 Note that these indexes are physical CPUs on the host.  When
 configuring multiple guests, you probably don't want to overlap these
@@ -228,3 +298,6 @@ command:
         ----- Entering VM 0 Shell -----
         *** Booting Zephyr OS build v2.6.0-rc1-324-g1a03783861ad  ***
         Hello World! acrn
+
+
+.. _EHL: https://www.intel.com/content/www/us/en/products/docs/processors/embedded/enhanced-for-iot-platform-brief.html