TLB Timing interface not match with MMIO instruction fetch in IFU

[Guanghui Hu]

Issue Description:

I have a question about the timing interface for accessing the TLB during MMIO instruction fetches. Although I may not have fully understood the code which is linked in below url. I'm unsure about the behavior of the IFU's (Instruction Fetch Unit) MMIO fetch state machine when checking the TLB's return result.
https://github.com/OpenXiangShan/XiangShan/blob/8fae59bba57fd80fcd1d85aadbf87895b97d167a/src/main/scala/xiangshan/frontend/IFU.scala#L590C1-L590C1

It seems that whenever the condition req.valid && !resp.bits.miss is observed within the same cycle, the state will change. However, the TLB requires at least one cycle to return the correct resp.bits.miss, even though the MMIO TLB is blocking. Suppose a TLB request for a specific MMIO is miss, due to the delayed response by one cycle, the MMIO state machine sees miss == false.B. However, the data in the TLB interface is incorrect.

Personally, I wonder if using resp.valid would be better than resp.bits.miss in this case.

[Xim]

Seems to be a problem. Thanks for your report!