At what instant process starts using kernel stack ?

[Mayuresh <infinite.questions@xxxxxxxxx>]

A process has both user space stack and kernel space stack. When a system call happens, all parameters are either copied to stack or in registers and using exception it switches to kernel. As from asmlinkage, I understand parameters are passed onto stack. So once in kernel space, all parameters are poped from stack, which is user stack.

My query is when does a process starts using kernel stack ?