Re: x86 swiotlb questions

["Jan Beulich" <jbeulich@novell.com>]

>Here's a thought: if the highmem DMA requests come from *only* the blkdev
>subsystem, then perhaps we could use its highmem bounce buffer (I think that
>still exists?). We turn that off on Xen right now, but we could re-enable
>it, leading to a slightly odd 'double bounce buffer': the first taking us
>from high pseudophysical memory to low pseudophysical memory, and the second
>taking us from high machine memory to low machine memory. If we can ensure
>only lowmem requests get to the swiotlb then a lot of the Xen diffs go away.
>I'm not sure whether we might get DMA requests from high memory from things
>other than block devices though, nor whether all block devices would
>actually pass through the blkdev bounce buffer code.

Proving that no driver ever passes highmem pages into any of the DMA ops should
be at least difficult, but would be needed since native i386 has no requirement that
dma_map_sg() or dma_map_page() be called only with non-highmem pages. I am
therefore not thinking this is a realistic option.

Jan