Jason Gunthorpe wrote: > On Fri, Mar 21, 2025 at 12:52:30AM +0100, Marek Szyprowski wrote: > > > Christoph's vision was to make a performance DMA API path that could > > > be used to implement any scatterlist-like data structure very > > > efficiently without having to teach the DMA API about all sorts of > > > scatterlist-like things. > > > > Thanks for explaining one more motivation behind this patchset! > > Sure, no problem. > > To close the loop on the bigger picture here.. > > When you put the parts together: > > 1) dma_map_sg is the only API that is both performant and fully > functional > > 2) scatterlist is a horrible leaky design and badly misued all over > the place. When Logan added SG_DMA_BUS_ADDRESS it became quite > clear that any significant changes to scatterlist are infeasible, > or at least we'd break a huge number of untestable legacy drivers > in the process. > > 3) We really want to do full featured performance DMA *without* a > struct page. This requires changing scatterlist, inventing a new > scatterlist v2 and DMA map for it, or this idea here of a flexible > lower level DMA API entry point. > > Matthew has been talking about struct-pageless for a long time now > from the block/mm direction using folio & memdesc and this is > meeting his work from the other end of the stack by starting to > build a way to do DMA on future struct pageless things. This is > going to be huge multi-year project but small parts like this need > to be solved and agreed to make progress. > > 4) In the immediate moment we still have problems in VFIO, RDMA, and > DRM managing P2P transfers because dma_map_resource/page() don't > properly work, and we don't have struct pages to use > dma_map_sg(). Hacks around the DMA API have been in the kernel for > a long time now, we want to see a properly architected solution. So I am late to this party, but after watching a "modest" proposal of a DMABUF pfn exporter bounce off the DRM community due to long standing pain points with scatterlist abuse [1], it is clear to me that a new DMA mapping API is in the critical path for PCI Device Security (Confidential Computing: TEE I/O). Specifically, the confidential computing problem of how to coordinate the conversion of assigned devices from shared-world to private-world (including private device MMIO and DMA), needs a "non-scatterlist" "struct-page-less" mapping contract to describe those resources. I concede the point that there are gaps missing between this proposal and the end state needed for PCI Device Security. However, it seems to be a case of "violent agreement" that some of the benefits of this proposal only arrive with future work. So this is a necessary first step. For my part, I plan to pull this series into a cross-vendor staging tree for device-security topics [2] so that the PCI Device Security community can get started on everything that needs to build on top of this. [1]: http://lore.kernel.org/20250107142719.179636-1-yilun.xu@xxxxxxxxxxxxxxx [2]: https://web.git.kernel.org/pub/scm/linux/kernel/git/devsec/tsm.git/
