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SubjectRe: [RFC PATCH 0/3] allow fallback to swiotlb on hw iommu init failures
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On Fri, 23 Oct 2009 09:39:23 -0700
Chris Wright <chrisw@sous-sol.org> wrote:

> > The concept sounds fine but the third patch doesn't look correct.
> >
> > Seems that the third patch doesn't take into account enabling both hw
> > iommu and swiotlb (Calgary does and I guess VT-d and AMD need that
> > too).
>
> VT-d isn't using swiotlb. Nor is AMD, although I think it will pick up
> no_iommu on passthrough (seems like it would benefit from swiotlb in
> that case).

I think that they need swiotlb for the same reason why Calgray needs
it. IIRC, someone in VT-d camp said so.


> > Also (iommu_detected && !dma_ops) trick doesn't work for
> > Calgary, IIRC.
>
> Yes, I think you are right. I had stared at the calgary code and thought
> it would DTRT due to calgary's use of no_iommu as fallback, but instead
> it will never pick up swiotlb_dma_ops.

Note that Calgary comment 'falling back to no_iommu' is misleading. It
actually falls back to swiotlb or nommu properly.

Calgary doesn't set to dma_ops to calgary_dma_ops so it doesn't need
to pick up swiotlb_dma_ops.


> The calgary shouldn't even need to be manually setting up
> nommu_dma_ops.

Yeah, but it needs because of how the dma startup code works.


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