Takashi's Notes

09-status-and-trap-architecture

4 min read

Status and Trap Architecture

Two kinds of Traps:

  1. Exceptions
  2. Interrupts
  • System call (ecall)
  • Program Error
    • Illegal Instruction, Alignment Error
  • Device Interrupts
    • Happens in Supervisor or User mode

Trap Handling

  • stvec is a CSR that contains the address of handler code.
  • kernelvec handles traps in supervisor mode
  • uservec handles traps in user mode

Supervisor Mode Trapping

  • sstatus is the CSR that holds the status of interrupts in supervisor mode.
    • sie is a bit that holds whether or not interrupts are enabled/disabled.
    • spie is previous interrupts enabled.
    • spp is previous privilege level the interrupt happened in, with 0 = User and 1 = Supervisor.

The kernel sets sie to prevent interrupts to the core.

When a trap occurs, if interrupts are disabled, the trap will wait until later for processing. If interrupts are enabled, then the handler will handle it right away.

The hardware then stores registers to use in the trap:

  1. pc is saved in sepc.
  2. stvec is saved in pc.
  3. scause gets register info about the interrupt:
    • 1 = Timer interrupt
    • 8 = System call
    • 9 = External Device
    • anything else = program exception
  4. stval stores other info about the fault
    • the address or instruction of the trap
    • a faulting memory address during page faults
    • if an illegal instruction, the failing instruction
  5. sstatus.SPP is assigned the previous mode (0 = user, 1 = supervisor)
  6. sstatus.SPIE is assigned sstatus.SIE (previous interrupts enabled)
  7. sstatus.SIE is set to 0 to disable interrupts
  8. mode is set to supervisor.

Then the trap executes, and returns to the handler. The handler then restores context back to the running code.

Once the supervisor is done, it calls sret to do the following:

  1. sstatus.SIE is assigned sstatus.SPIE
  2. mode is assigned sstatus.SPP
  3. pc is assigned sepc

To undo the storage done.

Machine Mode Trapping

Similar to supervisor mode, except for mpp

  • mstatus is the CSR that holds the status of interrupts in machine mode.
    • mie is a bit that holds whether or not interrupts are enabled/disabled.
    • mpie is previous interrupts enabled.
    • mpp is previous privilege level the interrupt happened in, with 00 = User and 01 = Supervisor, and 11 = machine mode

The only interrupt is for the timer. Every other interrupt goes to supervisor mode.

Interrupts are always enabled in machine mode

And on an timer interrupt:

  • Force a software interrupt to supervisor mode
  • re-enable interrupts
  • returns to the interrupted code

mtvec contains the address of the handler

The hardware also stores registers from the interrupted code:

  1. pc is saved in mepc.
  2. mtvec is saved in pc.
  3. mcause and mtval are ignored
  4. mstatus.SPP is assigned the previous mode (00 = user, 01 = supervisor, 11 = machine)
  5. mstatus.SPIE is assigned mstatus.SIE (previous interrupts enabled)
  6. sstatus.SIE is set to 0 to disable interrupts
  7. mode is set to machine.

The handler will then cause an interrupt at the supervisor level, and then mret to restore registers

  1. mstatus.SIE is assigned mstatus.SPIE
  2. mode is assigned mstatus.SPP
  3. pc is assigned mepc

Status values

seie indicates whether device interrupts are on in supervisor mode stie indicates whether time interrupts are on in supervisor mode ssie indicates whether software interrupts are on in supervisor mode sip indicates whether or not an interrupt is pending, 0 = no interrupt.

Hardware Delegation Registers

  • medeleg is for delegating exceptions to supervisor mode
  • mideleg is for delegating interrupts to supervisor mode.

Medeleg

medeleg has a bunch of bytes:

  • STORE/AMO page fault
  • Load page fault
  • Instruction page fault
  • ecall m-mode
  • ecall s-mode
  • ecall u-mode
  • STORE/AMO access fault
  • STORE/AMO misaligned
  • Load access fault
  • Load misaligned
  • breakpoint
  • illegal instruction
  • Instr. access fault
  • Inst. Misaligned

medeleg is set with csrw medeleg, $val

Mideleg

mideleg stores interrupt info:

  • Device interrupt in m-mode
  • Device interrupt in s-mode
  • Timer interrupt in m-mode
  • Timer interrupt in s-mode
  • Software interrupt in m-mode
  • Software interrupt in s-mode

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