doc: add instruction of Open vSwitch

Add instruction to show user how to enable Open vSwitch on ACRN.

Tracked-On: #3132
Signed-off-by: ruix.li <ruix.li@intel.com>

Author: lirui34

doc/tutorials/images/default-acrn-network.png

@@ -0,0 +1,155 @@
+.. _open_vswitch:
+
+How to enable OVS in ACRN
+#########################
+Hypervisors need the ability to bridge network traffic between VMs
+and with the outside world. This tutorial describes how to
+use `Open Virtual Switch (OVS)
+<https://www.openvswitch.org/>`_ bridge in ACRN for this purpose.
+
+.. note::
+   OVS is provided as part of the ``service-os``
+   bundle.  Use ClearLinux OS version ``29660`` or higher.
+
+What is OVS
+***********
+Open vSwitch (OVS) is an open-source implementation of
+a distributed virtual multilayer switch that provides a switching
+stack for hardware virtualization environments. OVS supports multiple
+protocols and standards used in computer networks. For more detailed
+OVS information, please refer to `what-is-ovs
+<http://docs.openvswitch.org/en/latest/intro/what-is-ovs/#what-is-open-vswitch>`_.
+
+Why OVS
+*******
+Open vSwitch is targeted at multi-server virtualization deployments,
+a landscape not well suited for ACRN's built-in L2 switch (the `Linux bridge
+<https://wiki.linuxfoundation.org/networking/bridge>`_).
+These environments are often characterized by highly dynamic end-points,
+the maintenance of logical abstractions, and (sometimes) integration with
+or offloading to special purpose switching hardware.
+For more reasons about why Open vSwitch is used, please refer to `why-ovs
+<http://docs.openvswitch.org/en/latest/intro/why-ovs/>`_.
+
+.. _enable_ovs_in_ACRN:
+
+How to enable OVS in ACRN
+*************************
+The OVS service is included with the Clear Linux ``service-os`` bundle.
+
+After booting the ACRN Service OS, disable the Clear Linux
+autoupdate feature before setting up the OVS bridge to
+prevent autoupdate from restoring the default bridge after
+a system update::
+
+   # swupd autoupdate --disable
+
+You can then start the OVS service with the command::
+
+   # systemctl start openvswitch
+
+To start OVS automatically after a reboot, you should also use this command::
+
+   # systemctl enable openvswitch
+
+The default ``acrn-br0`` bridge is created by the Service VM ``systemd`` and
+supports the User VM network.
+
+.. figure:: images/default-acrn-network.png
+   :align: center
+
+   Default ACRN Network
+
+How to use OVS bridge
+*********************
+#. Disable acrn network configuration::
+
+   # cd /usr/lib/systemd/network/
+   # mv 50-acrn.network 50-acrn.network_bak
+
+#. Modify ``50-eth.network`` to enable DHCP on OVS bridge
+
+   .. code-block:: none
+
+      [Match]
+      Name=ovs-br0
+
+      [Network]
+      DHCP=ipv4
+
+#. Create OVS bridge and ``tap1`` network interface::
+
+   # ovs-vsctl add-br ovs-br0
+   # ip tuntap add dev tap1 mode tap
+   # ip link set dev tap1 down
+   # ip link set dev tap1 up
+
+#. Add ``eno1``, ``tap1`` into OVS bridge::
+
+   # ovs-vsctl add-port ovs-br0 eno1
+   # ovs-vsctl add-port ovs-br0 tap1
+
+#. Modify ``launch_uos.sh`` script to enable ``tap1`` device before launching the User VM:
+
+   .. code-block:: none
+
+      # sed -i "s/virtio-net,tap0/virtio-net,tap1/" /usr/share/acrn/samples/nuc/launch_uos.sh
+
+   .. note::
+      If you set up the User VM via `acrn_quick_setup.sh
+      <https://raw.githubusercontent.com/projectacrn/acrn-hypervisor/master/doc/getting-started/acrn_quick_setup.sh>`_,
+      then replace ``/usr/share/acrn/samples/nuc/launch_uos.sh`` with ``/root/launch_uos_<version>.sh``
+      in ``sed`` command above.
+
+#. The User VM and Service VM network will work after rebooting the host via ``ovs-br0``
+
+Example for VLAN network based on OVS in ACRN
+*********************************************
+We will use the OVS bridge VLAN feature to support network isolation
+between VMs. :numref:`ovs-example1` shows an example with four VMs in two hosts,
+with the hosts directly connected by a network cable. The VMs are interconnected
+through statically configured IP addresses, and use VLAN id to put VM1 of
+HOST1 and VM1 of HOST2 into a VLAN. Similarly, VM2 of HOST1 and VM2 of
+HOST2 are put into a VLAN. In this configuration, the VM1s can communicate with each other,
+and VM2s can directly communicate with each other, but VM1s and VM2s cannot connect.
+
+.. figure:: images/example-of-OVS-usage.png
+   :align: center
+   :name: ovs-example1
+
+   An example of OVS usage in ACRN
+
+Follow these steps to set up OVS networks on both HOSTs:
+
+#. Set up ``ovs-br0`` instead of ``acrn-br0``, (refer to the the previous section
+   :ref:`enable_ovs_in_ACRN` for details).
+
+#. Add ``eno1``, ``tap<VM number>`` into OVS bridge:
+
+   .. code-block:: none
+
+      # ovs-vsctl add-port ovs-br0 eno1
+      # ovs-vsctl add-port ovs-br0 tap1 tag=101
+      # ovs-vsctl add-port ovs-br0 tap2 tag=102
+      # sed -i "s/virtio-net,tap0/virtio-net,tap1/" <1st launch_uos script>
+      # sed -i "s/virtio-net,tap0/virtio-net,tap2/" <2nd launch_uos script>
+      # reboot
+
+#. Configure the static IP address on both HOSTs and it's VMs::
+
+   # <HOST_1 Service VM>:
+   # ifconfig ovs-br0 192.168.1.100
+   # <HOST_1 User VM1>:
+   # ifconfig enp0s4 192.168.1.101
+   # <HOST_1 User VM2>:
+   # ifconfig enp0s4 192.168.1.102
+   #
+   # <HOST_2 Service VM>:
+   # ifconfig ovs-br0 192.168.1.200
+   # <HOST_2 User VM1>:
+   # ifconfig enp0s4 192.168.1.201
+   # <HOST_2 User VM2>:
+   # ifconfig enp0s4 192.168.1.202
+
+#. After that, it will succeed to ``ping`` from VM1 of HOST1 to VM1 of HOST2,
+   but fail to ``ping`` from VM1 of HOST1 to VM2 of HOST2.