After Excuting WFI, SPIKE and NEMU Lose Synchronization about nemu HOT 6 CLOSED

@poemonsense Do you have any idea about this?

from nemu.

I have never tested the nemu-diff-with-spike. However, I have a general concern of the position of wfi. Can you have a try moving wfi to the second instruction?

For example,

nop
wfi
li a1, 0x20240603
.word 0x6b # Hit Good Trap

from nemu.

I tried. But there is no difference.

This small program was extraced from a large program (xvisor). I think it is representative enough.

from nemu.

It seems that I found the problem.

The previous workable spike-so is built from pxk27/riscv-isa-sim and it removed some wfi logic at this commit.

      // debug mode wfis must nop
      if (unlikely(in_wfi && !state.debug_mode)) {
        throw wait_for_interrupt_t();
      }

But current spike-so is built from OpenXiangShan/riscv-isa-sim, which keeps the wfi logic.

This wfi code would throw an exception, skip pc+1 then quit step(), in order to handle interrupts outside. But for difftest ref so, there is no handler outside the step().

Now we have two way to solve this problem:

• let dut (XiangShan and NEMU) require to skip wfi instruction on ref side.
• remove wfi logic from spike step().

from nemu.

It seems that I found the problem.

The previous workable spike-so is built from pxk27/riscv-isa-sim and it removed some wfi logic at this commit.

      // debug mode wfis must nop
      if (unlikely(in_wfi && !state.debug_mode)) {
        throw wait_for_interrupt_t();
      }

But current spike-so is built from OpenXiangShan/riscv-isa-sim, which keeps the wfi logic.

This wfi code would throw an exception, skip pc+1 then quit step(), in order to handle interrupts outside. But for difftest ref so, there is no handler outside the step().

Now we have two way to solve this problem:

• let dut (XiangShan and NEMU) require to skip wfi instruction on ref side.
• remove wfi logic from spike step().

Use something like #ifdef CONFIG_DIFFTEST?

from nemu.

Use something like #ifdef CONFIG_DIFFTEST?

I agree. I will open a PR at spike repo.

from nemu.