DMA, TTM and memory encryption

[Thomas Hellström (VMware)]

Hi, Christoph,

It would be good to revisit this to see if we could set a direction for 
supporting user-space mapping of dma-coherent memory with TTM.

I think in the end, what TTM needs is a DMA interface similar to the one 
outlined here:

https://lore.kernel.org/lkml/b811f66d-2353-23c6-c9fa-e279cdb0f832@shipmail.org/

That could without too much effort be implemented for all architectures 
TTM supports and also for those weird architectures you've described 
that need special treatment, it would probably suffice with a more 
elaborate definition of the dma_pfn_t together with the dma coherent 
fault work you were suggesting earlier.

Regardless, once this patchset is in,

https://lore.kernel.org/lkml/20200304114527.3636-1-thomas_os@shipmail.org/

a short term solution would be to export the dma_pgprot() function and 
have TTM use it to set the pgprot for dma-coherent memory.

What do you think about this?

Thanks,

Thomas