ACRN Project Technical Community Meeting Minutes - 10/31/2018
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Q: What’s the difference b/w DMA address and HPA address? A: In Guest, guest driver use GPA as DMA target address. DMA Remapping unit will translate the GPA to HPA, so that the device can access the memory directly.
Q: Why x86 devices can't access any memory address rather than only DMA addresses? A: Not sure if understand the question correctly. DMA is the feature of computer systems that allows devices to access main system memory independent of the CPU.
Q: Will all the devices under one host bridge will be collectively assigned to a single VM? A: No. The case when a group of devices needs to be collectively assigned to a single domain, is that when these devices may have the same source-id (BDF) in the DMA requests.
If they share the same source id, DMA remapping hardware can’t uniquely identify the source of the requests. So they need to be assigned collectively.
Q: What is iommu group? devices of same group can only be passthrough to one domain. A: The same answer as the previous question.
Q: what should I do when I want pass though an device using DMA to VM. need modify bios? guest OS driver?
A: First, need to make sure that VT-d feature has been enabled in BIOS.
No need to modify guest OS driver. On ACRN, you can follow the steps to pass-through a PCI device (USB host controller) to a VM.
Q: When Guest OS issues an DMA request, How ACRN processes it, what is the detail sequence?
A: The DMA requests are initiated in guest OS by device driver.
If the device has been passed-through to a VM. The DMA operations can be completed without the intervention of hypervisor.
The DMA Remapping Unit will do the DMA address translation from GPA to HPA automatically.
Hypervisor is invoked only when guest software accesses protected resources that will impact system functionality and isolation (such as PCI configuration space accesses, interrupt management, etc.).
Q: IS VT-d enabled for graphics in ACRN?
A: When GVT-g (, which enables graphics sharing b/t multiple VM) is used as the graphics solution, the IOMMU for graphics device is not enabled in ACRN.
By default, GVT-g is enabled, so by default, IOMMU for graphics device is not enabled by default.
3. All: Community open discussion.
4. Next meeting agenda proposal:
Kabylake (Core) NUC (with serial port): NUC7i5BNH
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