Are there any details around how the round-trip and exchange of data (CPU<->GPU) is implemented in order to not be a big (partially-hidden) performance hit?

e.g. this code seems like it would entirely run on the CPU?

    print!("Enter your name: ");
    let _ = std::io::stdout().flush();
    let mut name = String::new();
    std::io::stdin().read_line(&mut name).unwrap();

But what if we concatenated a number to the string that was calculated on the GPU or if we take a number:

    print!("Enter a number: ");
    [...] // string number has to be converted to a float and sent to the GPU
    // Some calculations with that number performed on the GPU
    print!("The result is: " + &the_result.to_string()); // Number needs to be sent back to the CPU

Or maybe I am misunderstanding how this is supposed to work?