Before qemu 1.0 there was some discussion, and several patch series proposed for adding support for guest visible IOMMU emulation to qemu.
No agreement was reached on how to do this, but a patch series did go in preparation - a conversion of most existing PCI devices to use helper functions for their DMA which could be converted to do IOMMU translation in one place. This patch series is a further set of preliminaries, generalizing these wrapper functions to support non-PCI functions, and using this to convert several more drivers. It also introduces the concept of a DMAContext, describing a DMA address space. With this series the only one supported is NULL, meaning "no translation". However, this provides a place that we can put the necessary callback information to support IOMMU translation. If this series doesn't attract too many screams, I'll fold in feedback and repost for inclusion.
