However, stuff like WebAssembly and other sandboxing methods can be used to leverage two processes into the same address space. Your filesystem driver then simply lives as a module in address space and it's a normal process. The IPC turns into a simple jump using a pointer value provided by the kernel (which depending on the trust level can provide parameter validation or can be a plain pointer to the correct function).
However, stuff like WebAssembly and other sandboxing methods can be used to leverage two processes into the same address space. Your filesystem driver then simply lives as a module in address space and it's a normal process. The IPC turns into a simple jump using a pointer value provided by the kernel (which depending on the trust level can provide parameter validation or can be a plain pointer to the correct function).