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