Re: [PATCH 3/4] dma-buf: add support for mapping with dma mapping attributes

[Christoph Hellwig]

On Mon, Jan 21, 2019 at 11:44:10AM -0800, Liam Mark wrote:
> The main use case is for allowing clients to pass in 
> DMA_ATTR_SKIP_CPU_SYNC in order to skip the default cache maintenance 
> which happens in dma_buf_map_attachment and dma_buf_unmap_attachment. In 
> ION the buffers aren't usually accessed from the CPU so this allows 
> clients to often avoid doing unnecessary cache maintenance.

This can't work.  The cpu can still easily speculate into this area.
Moreover in general these operations should be cheap if the addresses
aren't cached.