关于NEMU和NutShell的`pmpaddr`对齐问题

[LuLuji04]

Describe the bug

当使用NEMU作为NutShell的参考时，由于NutShell未实现 pmpaddr4-15，NutShell抛出了非法指令异常，但是NEMU却没有触发异常。

这是NutShell的运行log

这是二者的差异，由于NEMU没有触发异常，其mepc没有更新

Expected behavior

当使用NEMU作为NutShell的参考时，由于NutShell未实现 pmpaddr4-15，NEMU应该在 M-mode 下访问该 CSR 应抛出非法指令异常。

To Reproduce

在NEMU中编译

make riscv64-nutshell-ref_defconfig
make -j

在NutShell中运行以下程序test.zip，以刚编译的NEMU为参照

./build/emu -i /<path-to>/test/build/test-riscv64-nutshell.bin  --diff /<path-to>/NEMU/build/riscv64-nemu-interpreter-so

Environment

• NutShell branch: master
• NutShell commit id: fc12171d929e7e589fab9f794ab63ce12e6c594e
• NEMU commit id: 4a24b77

[NewPaulWalker]

NEMU应该在 M-mode 下访问该 CSR 应抛出非法指令异常。

I disagree with this statement.

I found the following description in the RISC-V PMP specification:

Implementations may implement zero, 16, or 64 PMP entries; the lowest-numbered PMP entries must be implemented first. All PMP CSR fields are WARL and may be read-only zero.

Therefore, there is no such thing as an implementation that only implements 4 pmpaddr registers. NEMU actually does the same. In https://github.com/OpenXiangShan/NEMU/blob/master/configs/riscv64-nutshell-ref_defconfig#L29 , 16 pmpaddr registers are still implemented, but only 4 of them are active; the rest are read-only zeros: https://github.com/OpenXiangShan/NEMU/blob/master/src/isa/riscv64/system/priv.c#L1773

I believe this might be a bug in NutShell.

[LuLuji04]

Got it! However, NutShell is no longer maintained, right? There are many mismatches between NutShell and the current version of NEMU.

[NewPaulWalker]

Got it! However, NutShell is no longer maintained, right? There are many mismatches between NutShell and the current version of NEMU.

I'm not very familiar with this, but currently OpenXiangShan/NEMU is mainly used for aligning with XiangShan, especially Kunminghu.