Factorization of Device Driver Code between Kernel and User Spaces

Device drivers, which are normally implemented as kernel code, pose stability problems since bugs in the drivers cause kernel crashes. Running device drivers as unprivileged user-level code has often been proposed as a solution to increase the robustness of the system. However, moving the entire driver to user space brings down the performance of the system. An alternative approach would be to retain the performance critical code in the kernel and move the less performance sensitive code to the user space. In this project, we propose a scheme for factorization of driver code based on performance. In our split driver, work that needs to be done fast such as device I/O and interrupt handling is retained in the kernel space. Work that is less common and can afford to be done slower such as configuration or statictics collection is moved to the user space. We implemented this scheme on PCnet32 network driver and measured the performance overhead incurred by moving some of the driver functions to user space. We found the performance overhead to be less than a factor of 2. Also, the performance of the critical operations retained in the kernel was not affected by this factorization.