| > unless you're doing weird shared-memory things in userspace Shared-memory things in userspace, i.e. buffers shared between 2 distinct user processes are no weirder than buffers that are shared between a user process and a kernel-mode driver. In both cases the buffers cannot be accessed by third parties. Moreover, the transfer of data between 2 processes through a shared buffer can be done without any context switch (which could be slow), if the 2 processes are executed on distinct cores. Therefore having the network device driver as a distinct process does not have to cause any reduction in performance, if the means for inter-process communication are chosen wisely. For any device driver that is implemented as a user process, the kernel can enable direct access to any I/O ports and memory-mapped I/O areas that are needed by the device, so the device driver can work in user mode without requiring any context switches. Such direct I/O access cannot be enabled for ordinary processes, because those are not trusted enough and also because the direct I/O access could be enabled only for a single process at a time. A dedicated device driver process solves both the trust problem and the multiple access problem equally well as a kernel-mode driver. |