Best way of implementing async/await in user space?

[ChocolateLoverRaj]

What I did so far:

• Interrupts (PS/2 keyboard, APIC timer, RTC timer)
• A way of running user space code and making syscalls
• Async functions which run in the kernel, and an executor for them

I want to design syscalls to make it well integrated with writing async code in the user space, but I'm not sure how to do this. I want to make it have good performance, which I think means minimizing the number of syscalls.

My plan

• Create syscalls such as "Receive keyboard scan code", and the user space sends a pointer to the kernel of where to store the scan codes.
• Create a sycall called "Wait for async" which will block until the kernel make some async event available, such as a keyboard or timer. This can be used by the user space future executor similar to how the kernel uses the hlt instruction to wait for something to happen.

Issues with my plan

• Is it a security risk to have the kernel write to a pointer given by the kernel? Is it ok if I make sure the memory is only the user space's memory and not some kernel memory and handle any page faults?
• How would I deal with the kernel pushing a key event at the same time as the user space popping a key event? In the kernel I can use a ArrayQueue but how would I do this if both the kernel and user space are accessing the same memory?
• Should I instead create multiple syscalls for the keyboard input, "Watch for keyboard inputs" and "Poll keyboard inputs", and have the user space do a syscall every time it polls keyboard input and then "Wait for async" if the kernel returns nothing?

Should I do something completely different from my current plan? Are there other problems with my plan? Are there better methods of doing this? I'm looking for suggestions.