Re: [PATCH v7 07/12] dma-mapping: introduce dma_has_iommu()

[Dan Williams <dan.j.williams@intel.com>]

On Sat, Oct 7, 2017 at 4:08 AM, David Woodhouse <dwmw2@infradead.org> wrote:
> On Fri, 2017-10-06 at 16:15 -0700, Dan Williams wrote:
>>
>> Right, crossed mails. The semantic I want is that the IOVA is
>> invalidated / starts throwing errors to the device because the address
>> it thought it was talking to has been remapped in the file. Once
>> userspace wakes up and responds to this invalidation event it can do
>> the actual unmap to make the IOVA reusable again.
>
> So basically you want to unmap it by removing it from the page tables
> and flushing the IOTLB, but you want the IOVA to still be reserved.
>
> The normal device-facing DMA API doesn't give you that today. You could
> do it with the IOMMU API though — that one does let you manage the IOVA
> space yourself. You don't want that IOVA used again? Well don't use it
> as the IOVA in a subsequent iommu_map() call then :)

Ah, nice. So I think I'll just add a dma_get_iommu_domain() so the
dma_ops implementation can exclude identity-mapped devices, and then
iommu_unmap() does the rest. Thanks for the pointer.
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