Irrational exception return behavior

[iankuan]

Let's take 04-Multitasking for example. I provide three doubts on exception return.

• First, Is it correctly to use assembly instruction control to replace exception return? In my opinion, control is only used when we want to change the state from privileged thread mode to unprivileged thread mode. We would change the state from handler mode to unprivileged mode when first task is created.

  /*context_switch.S*/
  activate:
  ...
  /* switch to process stack */
  msr psp, r0
  mov r0, #3
  msr control, r0
  isb
  ...

• Second, I can not figure it out why we need to change the state from handler mode to unprivileged thread mode before we exception return. We can just launch exception return directly.

  /*context_switch.S*/
  activate:
  ...
  /* switch to process stack */
  msr psp, r0
  mov r0, #3
  msr control, r0
  isb
  
  /* load user state */
  pop {r4, r5, r6, r7, r8, r9, r10, r11, lr}
  
  /* jump to user task */
  bx lr

• Third, I think it is not enough to deal with the register, "psr" by msr psr_nzcvq, ip. If the interrupt occurred when we are in IT block, It would enable the flag in "ipsr". But we could not restore the state by mrs because of the specification. However, to my best of my knowledge, It can only be restored completely by exception return.

  /*context_switch.S*/
  svc_handler:
   ...
  /* load kernel state */
  pop {r4, r5, r6, r7, r8, r9, r10, r11, ip, lr}
  msr psr_nzcvq, ip
  ...

By the way, I'm working on these problem. I would send the PR recently.

[iankuan]

Sorry about the third point which I mention. I misunderstand this situation. In that case, the claim is correct above but not in this case. We only use svc and we can control when it would happen, so we don't have to worry about ipsr.

[iankuan]

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