On Wed, 19 Apr 2023 23:19:22 -0700 Christoph Hellwig wrote: > > 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. Yeah, this one is "for ever". > > 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. Okay, that simplifies various aspects. We'll just used dma-bufs from the start in the new APIs.
