To make scatter/gather go fast, you either spend a lot of effort caching descriptor lists for pinned buffers (because you expect to see them often), or heavily

To make scatter/gather go fast, you either spend a lot of effort caching descriptor lists for pinned buffers (because you expect to see them often), or heavily optimising your VM's v2p translation machinery, or some combination of the two. And then you wind up discovering that you the driver writer aren't actually trusted and so you need to insert at least one if not several IOMMUs between the peripheral and the memory(ies) that they may access, managed by another software component in a different address space. Then someone asks you to make all of this work for clients in VMs. At which point you start wondering why you didn't just allocate a physically contiguous buffer at startup and copy to/from your client buffers using the CPU... Sorry for sounding triggered... 8)