If it's part of a register map or can be computed from a register, you can just map that section of the registers into that 4k page and then use the function ca

If it's part of a register map or can be computed from a register, you can just map that section of the registers into that 4k page and then use the function call to compute the return value without a context-switch. The hardware then becomes responsible for updating the value for you. The only thing you're left with is possible read latency, but that should be relatively small. That's my theory.