On Sat, Nov 23, 2019 at 09:39:08AM -0600, Tom Lendacky wrote: > Ideally, having a pool of shared pages for DMA, outside of standard > SWIOTLB, might be a good thing. On x86, SWIOTLB really seems geared > towards devices that don't support 64-bit DMA. If a device supports 64-bit > DMA then it can use shared pages that reside anywhere to perform the DMA > and bounce buffering. I wonder if the SWIOTLB support can be enhanced to > support something like this, using today's low SWIOTLB buffers if the DMA > mask necessitates it, otherwise using a dynamically sized pool of shared > pages that can live anywhere. I think that can be done relatively easily. I've actually been thinking of multiple pool support for a whіle to replace the bounce buffering in the block layer for ISA devices (24-bit addressing). I've also been looking into a dma_alloc_pages interface to help people just allocate pages that are always dma addressable, but don't need a coherent allocation. My last version I shared is here: http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma_alloc_pages But it turns out this still doesn't work with SEV as we'll always bounce. And I've been kinda lost on figuring out a way how to allocate unencrypted pages that we we can feed into the normal dma_map_page & co interfaces due to the magic encryption bit in the address. I guess we could have a fallback path in the mapping path and just unconditionally clear that bit in the dma_to_phys path.
