Re: [RFC PATCH 04/28] block: Never bounce dma-direct bios

[Jason Gunthorpe]

On Thu, Jun 20, 2019 at 10:12:16AM -0600, Logan Gunthorpe wrote:
> It is expected the creator of the dma-direct bio will ensure the
> target device can access the DMA address it's creating bios for.
> It's also not possible to bounce a dma-direct bio seeing the block
> layer doesn't have any way to access the underlying data behind
> the DMA address.
> 
> Thus, never bounce dma-direct bios.

I wonder how feasible it would be to implement a 'dma vec' copy
from/to? 

That is about the only operation you could safely do on P2P BAR
memory. 

I wonder if a copy implementation could somehow query the iommu layer
to get a kmap of the memory pointed at by the dma address so we don't
need to carry struct page around?

Jason