Driver signing is a killer issue on Windows; if you put your machine into dev/unsigned mode you get an ugly banner that can't be turned off.

Much easier to design the device to avoid that. E.g. by abusing USB-HID. The desktop USB missile launcher toy is USB HID, for example.

Arguably all these other subsystems shouldn't be in the Kernel either but that's a different topic :)

There are quite a few benefits to doing these things in userspace over the Kernel, not really necessarily just because of the code size:

- The code is much easier to write and debug, you just write code like you always would.

- Bugs don't have the possibility to taking down your entire system or introduce vulnerabilities

- Especially on Windows, everyone can do this without requiring an impossible to get driver signing certificate

In HFT user-space networking drivers have a long history - there is too much latency induced by switching from kernel to user space to handle networking.

> OpenOnload: A user-space network stack that intercepts socket calls to bypass the kernel network stack, accelerating standard socket operations for faster networking.

> Netmap: A framework providing a simple API for high-speed packet I/O in user space, bypassing much of the kernel overhead for efficient packet forwarding and filtering.

https://dysnix.com/blog/high-frequency-trading-infrastructur...