On Tue, 26 Nov 2019 19:45:27 +0100 Christoph Hellwig <hch@xxxxxx> wrote: > 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. Thanks Christoph! Thanks Tom! I will do some looking and thinking and report back. Regards, Halil
