doc: add guideline for RTVM workload design

a quick guideline document help RTOS designer/developer to design
RT workload based on RTVM

Signed-off-by: Jason Chen CJ <jason.cj.chen@intel.com>

Author: JasonChenCJ

doc/tutorials/rtvm_workload_design_guideline.rst

@@ -0,0 +1,45 @@
+.. _rtvm_workload_guideline:
+
+Real time VM application design guidelines
+##########################################
+
+An RTOS developer must be aware of the differences between running applications on a native
+platform and a real time virtual machine (RTVM), especially issues impacting real time
+performance. For example, a real time thread should avoid triggering any VM-Exit. If a VM-Exit
+is triggered, the developer must account for an additional margin of CPU cycles for the
+incremental runtime overhead.
+
+This document provides some application design guidelines when using an RTVM within the ACRN hypervisor.
+
+Run RTVM with dedicated resources/devices
+*****************************************
+
+For best practice, ACRN allocates dedicated CPU, memory resources, and cache resources (using Intel
+Cache Allocation Technology, aka CAT) to RTVMs. For best real time performance of I/O devices,
+we recommend using dedicated (pass-thru) PCIe devices to avoid VM-Exit at run time.
+
+.. note::
+   The configuration space for pass-thru PCI devices is still emulated and the accessing it will
+   trigger a VM-Exit.
+
+RTVM with virtio PMD (Polling Mode Driver) for I/O sharing
+**********************************************************
+
+If the RTVM must use shared devices, we recommend using PMD drivers that can eliminate the
+unpredictable latency caused by guest I/O trap-and-emulate access. The RTVM application must be
+aware that the packets in the PMD driver may arrive or be sent later than expected.
+
+RTVM with HV Emulated Device
+****************************
+
+ACRN uses hypervisor emulated virtual UART (vUART) devices for inter-VM synchronization such as
+logging output, or command send/receive.  Currently, the vUART only works in polling mode, but
+may be extended to support interrupt mode in a future release. In the meantime, for better RT
+behavior, the RT application using the vUART shall reserve a margin of CPU cycles to accommodate
+for the additional latency introduced by the VM-Exit to the vUART I/O registers (~2000--3000 cycles
+per register access).
+
+DM emulated device (Except PMD)
+*******************************
+
+We recommend **not** using DM-emulated devices in a RTVM.