On Wed, Apr 19, 2023 at 09:45:06AM -0700, Jakub Kicinski wrote: > > Can you explain what the actual use case is? > > > > From the original patchset I suspect it is dma mapping something very > > long term and then maybe doing syncs on it as needed? > > In this case yes, pinned user memory, it gets sliced up into MTU sized > chunks, fed into an Rx queue of a device, and user can see packets > without any copies. How long is the life time of these mappings? Because dma_map_* assumes a temporary mapping and not one that is pinned bascically forever. > Quite similar use case #2 is upcoming io_uring / "direct placement" > patches (former from Meta, latter for Google) which will try to receive > just the TCP data into pinned user memory. I don't think we can just long term pin user memory here. E.g. for confidential computing cases we can't even ever do DMA straight to userspace. I had that conversation with Meta's block folks who want to do something similar with io_uring and the only option is an an allocator for memory that is known DMAable, e.g. through dma-bufs. You guys really all need to get together and come up with a scheme that actually works instead of piling these hacks over hacks.
