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You could use the shared ring buffers scheme to replace essentially all syscalls and similar things in any context. Think gpu drivers, memory controllers, etc. It could be a universal interface for all communication that has to go through some kind of expensive security barrier.

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With cross-core interrupts and user-mode interrupt handlers (as in some new intel cpus), you could even do something without polling (interrupt for submission) where the core user-mode code is running on _never_ context switches (obviously except for scheduling) and you just have a dedicated kernel core or cores off doing kernel things.

You can also use umwait on the next completion entry in the ring

yup, though that means you're wasting that core's compute; something with green threads where language runtime does a cross-core interrupt to submit syscall then continues execing other green threads until it gets a user interrupt for syscall completion would be pretty neat.

(ETA: and indeed looks like they're considering support for that in io_uring! https://lwn.net/Articles/869140/)

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