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 :)