NEMU does not trap when accessing unimplemented CSRs

[shinezyy]

In src/isa/riscv64/system/priv.c, csr_read always returns *src even if this CSR is not defined, and does not trap.

However, OpenSBI uses the trap behavior to decide whether an extension/feature is implemented: https://github.com/riscv-software-src/opensbi/blob/0a667542950ac108e609c2bdfed4c0df87363717/include/sbi/sbi_csr_detect.h#L17

When CONFIG_RV_SMSTATEEN is not enabled in NEMU, reading mstateen0 returns *src and does not trap. Then OpenSBI thinks NEMU implements SMSTATEEN

[cebarobot]

I think accessing mstateen0 without CONFIG_RV_SMSTATEEN would trap into EX_II.

First, without CONFIG_RV_SMSTATEEN, mstateen0 would not defined in CSR.h: https://github.com/OpenXiangShan/NEMU/blob/84c7e4068512c90bb6a45c4b42a9309f459998b9/src/isa/riscv64/local-include/csr.h#L318-L324

Then, csr_exist[4096] is initialized: https://github.com/OpenXiangShan/NEMU/blob/84c7e4068512c90bb6a45c4b42a9309f459998b9/src/isa/riscv64/system/priv.c#L41-L48

After that, NEMU will traped when trying to accessing NEMU:

https://github.com/OpenXiangShan/NEMU/blob/84c7e4068512c90bb6a45c4b42a9309f459998b9/src/isa/riscv64/system/priv.c#L1031-L1035

https://github.com/OpenXiangShan/NEMU/blob/84c7e4068512c90bb6a45c4b42a9309f459998b9/src/isa/riscv64/system/priv.c#L91-L99

Function csr_read() and csr_write() just do the read and write. Accessiblity checking is done in csrrw() function.

Do you have any tests that may cause this problem?

[shinezyy]

I think accessing mstateen0 without CONFIG_RV_SMSTATEEN would trap into EX_II.

First, without CONFIG_RV_SMSTATEEN, mstateen0 would not defined in CSR.h:

https://github.com/OpenXiangShan/NEMU/blob/84c7e4068512c90bb6a45c4b42a9309f459998b9/src/isa/riscv64/local-include/csr.h#L318-L324

Then, csr_exist[4096] is initialized:

https://github.com/OpenXiangShan/NEMU/blob/84c7e4068512c90bb6a45c4b42a9309f459998b9/src/isa/riscv64/system/priv.c#L41-L48

After that, NEMU will traped when trying to accessing NEMU:

https://github.com/OpenXiangShan/NEMU/blob/84c7e4068512c90bb6a45c4b42a9309f459998b9/src/isa/riscv64/system/priv.c#L1031-L1035

https://github.com/OpenXiangShan/NEMU/blob/84c7e4068512c90bb6a45c4b42a9309f459998b9/src/isa/riscv64/system/priv.c#L91-L99

Function csr_read() and csr_write() just do the read and write. Accessiblity checking is done in csrrw() function.

Do you have any tests that may cause this problem?

Running OpenSBI and difftest against Spike

[shinezyy]

Cannot be reproduced on current master (ec82526c0c69317cec4046e31)