# Understanding problem DMA remapping

## musv

Hi there, 

after a long time I'm updating my kernel to 3.0.1 and stumbled into some kernel options for paravirtualization I can't understand. The main problem for me seems to be that I never know if that option has to be used in the host or in the guest system.

```
Bus Options (PCI etc.)

[ ] Support for DMA Remapping Devices (EXPERIMENTAL) (DMAR)

[ ] Support for Interrupt Remapping (EXPERIMENTAL) (INTR_REMAP)
```

After some googling I think I understood some parts:

The guest system want's to get direct access to some devices of the host. Now there can be the situation where the guest uses the addresses of the devices in any other way. Therefore the addresses of the host devices have to be remapped to other memory area within the guest system. Is that correct?

On my machine I have the the following configuration:

1. Host system: Gentoo

2. Guest system: WinXP (VMWare)

Is the activation in my case useful? Does it mean the kernel provides the remapping functionality to VMWare and WinXP inside? Or would it only make sense if Linux would be the guest inside any other OS?

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## Aquous

DMAR is for the host. It allows guests direct access to PCI(e) devices by mapping the host's PCI address space directly into the guest memory.

However, the host must have Intel VT-d or AMD IOMMU extensions for PCI passthrough to be possible.

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## opotonil

I don't know if DMA remapping is supported by VMware. If you have interest on how to use it in qemu-kvm:

http://www.linux-kvm.org/page/How_to_assign_devices_with_VT-d_in_KVM

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## musv

Thanks so far. I guess, I'll activate it in that case. Maybe VMWare can get a benefit of it.

 *opotonil wrote:*   

> I don't know if DMA remapping is supported by VMware. If you have interest on how to use it in qemu-kvm:
> 
> http://www.linux-kvm.org/page/How_to_assign_devices_with_VT-d_in_KVM

 

Mainly I use WinXP within the VMWare for any stuff that isn't available on Linux, e.g. my tax declaration. It's not worth to me to put a lot of work into installing WinXP in Qemu. It's working that way in VMWare for some years now.

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