1.. _rtvm_workload_guideline:
2
3Real-Time VM Application Design Guidelines
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6An RTOS developer must be aware of the differences between running applications on a native
7platform and a real time virtual machine (RTVM), especially issues impacting real time
8performance. For example, a real time thread should avoid triggering any VM-Exit. If a VM-Exit
9is triggered, the developer must account for an additional margin of CPU cycles for the
10incremental runtime overhead.
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12This document provides some application design guidelines when using an RTVM within the ACRN hypervisor.
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14Run RTVM With Dedicated Resources/Devices
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17For best practice, ACRN allocates dedicated CPU, memory resources, and cache resources (using Intel
18Resource Directory allocation Technology such as CAT, MBA) to RTVMs. For best real time performance
19of I/O devices, we recommend using dedicated (passthrough) PCIe devices to avoid VM-Exit at run time.
20
21.. note::
22   The configuration space for passthrough PCI devices is still emulated and accessing it will
23   trigger a VM-Exit.
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25RTVM With Virtio PMD (Polling Mode Driver) for I/O Sharing
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28If the RTVM must use shared devices, we recommend using PMD drivers that can eliminate the
29unpredictable latency caused by guest I/O trap-and-emulate access. The RTVM application must be
30aware that the packets in the PMD driver may arrive or be sent later than expected.
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32RTVM With HV Emulated Device
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35ACRN uses hypervisor emulated virtual UART (vUART) devices for inter-VM synchronization such as
36logging output or command send/receive.  The vUART only works in polling mode, but
37may be extended to support interrupt mode in a future release. In the meantime, for better RT
38behavior, the RT application using the vUART shall reserve a margin of CPU cycles to accommodate
39for the additional latency introduced by the VM-Exit to the vUART I/O registers (~2000-3000 cycles
40per register access).
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42DM Emulated Device (Except PMD)
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45We recommend **not** using DM-emulated devices in an RTVM.
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