Re: [kvm-devel] kvm + IOMMU

From: David Brown
Date: Wed Jun 13 2007 - 14:52:24 EST

Hi David,
I am not an expert here, but I don't believe it would work without
changes to KVM. My understanding is that you use an IOMMU in this
fashion if you want to direct-map a device into a guest for devices that
do not have a local IOMMU-like functionality built in already. For
instance, perhaps you want to assign an off-the-shelf ethernet NIC to a
guest. The IOMMU would serve to translate between GPA and system based
DMA addresses. However, the hypervisor would really need to be involved
in the setup of this mapping on the IOMMU in the first place.

Okay its understandable that the initial setup of the mapping between
virtual and actual would be done by some OS (most-likely host).
However isn't the actual mapping when the guest starts and requests
devices supposed to be handled by hardware? I would think performance
wouldn't scale very well if the host OS had to maintain mappings and
translate addresses every time a guest requests access to a mapped

KVM (currently) virtualizes/emulates all components in the logical
"system" presented to the guest. It doesn't yet support the notion of
direct-mapping a physical component. I doubt you will have to wait too
long for someone to add this feature, however :) It's just not there
today (to my knowledge, anyway)

That's good to hear. :)

But to answer your question, when configured up like this the IO
subsystem in question should perform pretty close to native (at least in

Hopefully you mean that the hardware is handling the mapping so that
the host OS won't have to take the burden of mapping a bunch of
addresses all the time.

- David Brown
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