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 Resource Directory allocation Technology such as CAT, MBA) to RTVMs. For best real time performance of I/O devices, we recommend using dedicated (passthrough) PCIe devices to avoid VM-Exit at run time.

Note

The configuration space for passthrough PCI devices is still emulated and 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.