On 09.03.2026 16:13, Jason Gunthorpe wrote: > On Mon, Mar 09, 2026 at 05:05:02PM +0200, Leon Romanovsky wrote: >> Regarding failure on unsupported systems, I have tried more than once to >> make the RDMA fail when the device is known to take the SWIOTLB path >> in RDMA and cannot operate correctly, but each attempt was met with a >> cold reception: >> https://lore.kernel.org/all/d18c454636bf3cfdba9b66b7cc794d713eadc4a5.1719909395.git.l...@kernel.org/ > I think alot of that is the APIs used there. It is hard to determine > if SWIOTLB is possible or coherent is possible, I've also hit these > things in VFIO and gave up. > > However, DMA_ATTR_REQUIRE_COHERENCE can be done properly and not leak > alot of dangerous APIs to drivers (beyond itself). > > It is also more important now with CC systems, I think.
Jason is right. Indeed the rdma/uverbs case needs some extension to ensure that the coherent mapping is used, what is not possible now. This however doesn't mean that the DMA_ATTR_CPU_CACHE_OVERLAP is not needed for that use case too. I'm open to accept both. The only question I have is which name should we use? We already have DMA_ATTR_CPU_CACHE_CLEAN, while DMA_ATTR_CPU_CACHE_OVERLAP and DMA_ATTR_DEBUGGING_IGNORE_CACHELINES were proposed here. The last seems to be most descriptive. Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland
