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[Xen-devel] Re: [XEN-IOMMU] Proposal of DMA protection/isolation support

On 16/1/08 16:34, "Wei Wang2" <wei.wang2@xxxxxxx> wrote:

> 1) Before submitting io request to generic block layer, block backend
> driver will invoke GNTTABOPs to map granted remote pages .  However
> creating iommu mapping only in GNTTABOP not very enough because local
> pages will also be submitted to block layer and they should also be
> translated by iommu.

Local pages could always be mapped into the iommu, and only remote pages
would need to be temporarily mapped via the gnttab hooks.

> 2) When dma layer is invoked to prepare a bus address for native block
> device driver. It would be better to have a "hypervisor-aware" dma layer
> which always maps virtual address to bus address via hypercall.
> __gnttab_dma_map_page() might be a good place to trigger this new
> hypercall. Both remote and local pages can be mapped by this way.

An extra hypercall here will likely hurt performance.

 -- Keir

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