ChatGPT - Updating the mstatus during exception

[amichai-bd]

In the RISC-V architecture, handling an illegal instruction exception involves both hardware and software actions, particularly affecting the mstatus register among others. Here's how these updates are managed:

Hardware (HW) Perspective: Illegal Instruction Exception
When an illegal instruction is encountered, the RISC-V hardware (the CPU core itself) automatically initiates a sequence of actions to handle the exception. Specifically for the mstatus register, here are the key updates:

Trap Taken: Upon detecting an illegal instruction, the hardware takes a trap to machine mode (if machine mode is the highest privilege level supported or if the illegal instruction exception is not delegated to a lower privilege level).
mstatus Register Updates:
MPP (Machine Previous Privilege): The MPP bits in the mstatus register are updated to reflect the privilege level the hart was executing in before the trap. This ensures that when an MRET instruction is later executed to return from the exception handler, the hart can return to the correct privilege level.
MPIE (Machine Previous Interrupt Enable): The value of the MIE (Machine Interrupt Enable) bit is saved into MPIE. This preserves the interrupt enable state before the trap occurred.
MIE (Machine Interrupt Enable): This bit is cleared to 0, disabling interrupts in machine mode to ensure the exception handler can execute atomically without being preempted by another interrupt.
These actions are performed by the hardware automatically as part of the exception entry sequence, ensuring the system enters a known state from which software can safely manage the exception.

Software (SW) Perspective: Handling the Illegal Instruction Exception
Once the hardware has transitioned control to the exception handler (by jumping to the address specified in the mtvec register), software takes over to manage the exception. The software's responsibilities typically include:

Identifying the Exception Cause: Software reads the mcause register to determine the cause of the trap. For an illegal instruction, the mcause register will have a specific cause code.
Inspecting the Faulting Instruction: The mepc (Machine Exception Program Counter) register holds the address of the instruction that caused the trap. Software can use this to analyze the instruction that led to the exception.
Corrective Action: Depending on the nature of the illegal instruction and the system's requirements, software may attempt to correct the error (if possible), log the error, or terminate the offending process.
Updating mstatus (if needed): Before returning from the exception handler, software may modify mstatus or other CSRs as part of the exception handling process. For instance, software may choose to re-enable interrupts by setting the MIE bit if it was previously enabled (restoring the MPIE value back to MIE).
Returning from the Exception: Software executes an MRET instruction to return from the exception handler. This action restores the previous privilege level and interrupt enable state from MPP and MPIE, and jumps back to the instruction address following the one that caused the trap (or another specified location if the handler changes the mepc value).
In summary, the hardware automatically updates the mstatus register to manage the privilege level and interrupt enablement state upon entering an exception, ensuring a consistent and safe environment for the software to handle the exception. The software then has the responsibility to manage the exception, potentially adjust the system state, and eventually return control to the normal execution flow.

User
Please feel free to admit or fix any mistake you may have wrote.
Maybe everything you wrote is correct - but i am not sure i understand this part you wrote:

Updating mstatus (if needed): Before returning from the exception handler, software may modify mstatus or other CSRs as part of the exception handling process. For instance, software may choose to re-enable interrupts by setting the MIE bit if it was previously enabled (restoring the MPIE value back to MIE).

If the SW will update the "MIE" bit in mstatus during the exception handler we may cause issues like nested exceptions that we don't want to support in our design.

Are you sure there may be a case where SW would want to update the "MIE" bit in mstatus? what about the other bits in mstatus?

3 / 3

ChatGPT
You're right to question the advisability of manually updating the MIE bit in the mstatus register during exception handling, particularly concerning the potential for nested exceptions. My explanation could have been clearer and more precise on this matter, especially considering system design intentions that might wish to avoid nested exceptions.

Correction and Clarification:
In general, software directly modifying the MIE bit of the mstatus register during an exception handler is not a common practice due to the reasons you've mentioned, such as the risk of unintended nested exceptions. Typically, the MIE bit is cleared by hardware upon entering an exception to disable interrupts and prevent such nesting. Here's a more detailed correction and clarification:

Automatic Hardware Actions: Upon entering an exception, the hardware clears the MIE bit to disable interrupts and sets the MPIE bit to the previous value of MIE. This is done to ensure atomic handling of the exception without interruption.

Software's Role: During exception handling, software typically does not modify the MIE bit directly. Instead, software handles the cause of the exception, possibly updating relevant CSRs (other than mstatus.MIE) as needed for exception processing or system state management.

Returning from Exception: The MRET instruction, used to return from the exception, automatically restores the MIE bit to the value stored in MPIE. This ensures that if interrupts were enabled before the exception, they are re-enabled upon returning from the handler, thereby avoiding manual intervention by software in the MIE management during exception handling.

Regarding Other mstatus Bits: Other bits in the mstatus register, like MPRV (Modify Privilege) or MPP (Previous Privilege), are also primarily managed by hardware during trap entry and exit. Software usually doesn't modify these bits as part of standard exception handling. However, there might be specific system-level or context-switch operations where software manages these bits explicitly, but such cases are outside the normal scope of exception handling.

[amichai-bd]

@roman012285 - please read this (updating mstatus during exception