Re: [PATCH 9/9] x86/iommu: use dma_ops_list in get_dma_ops

[Muli Ben-Yehuda]

On Sun, Sep 28, 2008 at 09:13:33PM +0200, Joerg Roedel wrote:

> I think we should try to build a paravirtualized IOMMU for KVM
> guests.  It should work this way: We reserve a configurable amount
> of contiguous guest physical memory and map it dma contiguous using
> some kind of hardware IOMMU. This is possible with all hardare
> IOMMUs we have in the field by now, also Calgary and GART. The guest
> does dma_coherent allocations from this memory directly and is done.
> For map_single and map_sg 
> the guest can do bounce buffering. We avoid nearly all pvdma hypercalls
> with this approach, keep guest swapping working and solve also the
> problems with device dma_masks and guest memory that is not contigous on
> the host side.

I'm not sure I follow, but if I understand correctly with this
approach the guest could only DMA into buffers that fall within the
range you allocated for DMA and mapped. Isn't that a pretty nasty
limitation?  The guest would need to bounce-bufer every frame that
happened to not fall inside that range, with the resulting loss of
performance.

Cheers,
Muli
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