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riscv / riscv-cfi

This specification is integrated into the Priv. and Unpriv. specifications. This repo is no longer maintained. Please refer to the Priv. and Unpriv. specifications at https://github.com/riscv/riscv-isa-manual
https://lf-riscv.atlassian.net/browse/RVG-80
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If xLPE=0，should xPELP be cleared when executing xRET instruction? #233

Open Baixyzz opened 2 days ago

Baixyzz commented 2 days ago

In Zicfilp spec，we can see “An MRET or SRET instruction is used to return from a trap in M-mode or S-mode, respectively. When executing an xRET instruction, xPELP is set to NO_LP_EXPECTED.”From this description, we can't see any description about xLPE.If mseccfg.MLPE =0 and mstatus.MPELP =1,should mstatus.MPELP keep its original value or set to NO_LP_EXPECTED when executing mret instruction?

ved-rivos commented 2 days ago

The behavior specified is as follows:

An MRET or SRET instruction is used to return from a trap in M-mode or S-mode, respectively. When executing an xRET instruction, if the new privilege mode is y, then ELP is set to the value of xPELP if yLPE is 1; otherwise, it is set to NO_LP_EXPECTED; xPELP is set to NO_LP_EXPECTED.

The last statement states that xPELP is unconditionally set to NO_LP_EXPECTED

Baixyzz commented 2 days ago

The behavior specified is as follows:

An MRET or SRET instruction is used to return from a trap in M-mode or S-mode, respectively. When executing an xRET instruction, if the new privilege mode is y, then ELP is set to the value of xPELP if yLPE is 1; otherwise, it is set to NO_LP_EXPECTED; xPELP is set to NO_LP_EXPECTED.

The last statement states that xPELP is unconditionally set to NO_LP_EXPECTED

@ved-rivos Thank you very much for your answer.But when I use the latest version of spike to simulate the scenario I mentioned, I see that mstatus.MPELP remains LP_EXPECTED after executing the mret instruction.So I want to know if SPIKE's handling is wrong？

ved-rivos commented 1 day ago

The code in spike seems to set the MPELP unconditionally to NO_LP_EXPECTED. Do you know why the statement 4 is not taking effect?


1. if (ZICFILP_xLPE(prev_virt, prev_prv)) {
2.   STATE.elp = static_cast<elp_t>(get_field(s, MSTATUS_MPELP));
3. }
4. s = set_field(s, MSTATUS_MPELP, elp_t::NO_LP_EXPECTED);