Why use UPIntrFreeCell to manage global resources instead of UPSafeCell?

[Hlinbit]

After comparing the code in rCore and the code in rCore-Tutorial-Code-2022S, I notice that rCore uses UPIntrFreeCell to manage the global resources while the lab code uses UPSafeCell.
Why the change is necessary? In fact the document didn't give any explanation. And most confusing is that when we access the value in UPIntrFreeCell, the sie is read and some information is recorded in IntrMaskingInfo.
Can anyone help me ?

[wangrunji0408]

cc @wyfcyx