When CPU switches from Ring-3 to Ring-0, SS is also switched to Ring-0 stack. In a multi-processor environment, if there are N number of CPU's, does each CPU will have it's own Ring-0 stack?
Each hardware thread would require a seperate stack.
Note, the seperate stacks can be seperate areas within the same selector (with different esp/rsp) or within different selectors. This is an operating system design issue.