Question of a machine-level trap handler described in aia spec(5.2.2)

[ZeyueShen]

As shown in this figure, when the condition: if (mstatus.mpie == 1) is met in the above pseudo-code, the interrupt service program will be entered, but there is such a scenario:
a.set mie.mtie = 1 in M mode,
b.set mtimecmp = 0 ,which making mip.stip = 1 in M mode
c.Execute mret to change the privilege mode from M mode to S mode
At this point, an interrupt will cause a trap, but since mstatus.mie = 0, after the trap occurs, mstatus.mpie will also be equal to 0. This means that the condition mpie == 1 will never be met, thus rendering the interrupt unhandled. This scenario puzzles me greatly.

[jhauser-us]

@ZeyueShen, your question makes me realize that the AIA document should mention that software must be sure that mstatus.MPIE = 1 before executing an MRET to change to another privilege mode.

If we assume mstatus.MPIE = 1 before your step c, there is no problem. The MRET copies MPIE to MIE, and sets MPIE = 1. When the interrupt trap occurs, it then copies MIE to MPIE, and sets MIE = 0. Thus, MPIE is restored to 1 at the start of the trap handler.

[ZeyueShen]

Thanks