Comments (10)
aswaterman
commented on September 24, 2024
Yeah, this is benign. Setting xPIE=1 is meant to detect software bugs; it
should not matter what happens to the xPIE field after a yRET. You're
welcome to submit a pull request.
…On Fri, Jan 6, 2017 at 8:01 PM, Hongce Zhang ***@***.***> wrote:
Hi,
I wonder if there is a mismatch between the privilege-level spec and the
Spike simulator.
In the spec (Version 1.9.1 released on 11/04/16) page 22, the semantics of
xRET are described as follows,
The MRET, HRET, SRET, or URET instructions are used to return from traps
in M-mode, H-mode, S-mode, or U-mode respectively. When executing an xRET
instruction, supposing xPP holds the value y, yIE is set to xPIE; the
privilege mode is changed to y; xPIE is set to 1; and xPP is set to U.
Which means if we execute MRET in M-mode, and returns to say S-mode, MPIE
should be set to 1, but in the simulator, (riscv/insns/mret.h)
s = set_field(s, MSTATUS_MPIE, 0);
Could anyone tell if this is indeed a mismatch or just my misunderstanding?
Thank you!
Hongce
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zhanghongce
commented on September 24, 2024
Thank you for your reply! But I didn't get it why setting xPIE=1 can be used to detect software bugs. So it seems that we can treat the spec as saying xPIE can be anything after executing xRET, correct?
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sorear
commented on September 24, 2024
If a SRET is followed by a non-delegated trap into M-mode, the M-mode code can observe the value of SPIE which was written by SRET.
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aswaterman
commented on September 24, 2024
By "benign" I meant it doesn't create a security hole. It's still a bug and Spike needs to be fixed. I'll do so shortly.
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zhanghongce
commented on September 24, 2024
Thank you very much for your reply!
Sorry for bothering you again, but I just have one more question. It seems legal to execute an SRET instruction when in M-mode. Is this behavior desired?
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aswaterman
commented on September 24, 2024
It is legal. In practice, it is not likely to be especially useful, but allowing all instructions in M-mode simplifies testing.
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mainkar-rohit
commented on September 24, 2024
Hi Andrew,
It is legal to execute sret in M mode only if S mode is available right?
If S mode is not supported, isn't sret illegal?
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aswaterman
commented on September 24, 2024
That’s right.
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mainkar-rohit
commented on September 24, 2024
Thanks for clarifying. Is Spike always expected to have S & U mode?
It looks like currently these corresponding bits in misa are hardcoded to 1.
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aswaterman
commented on September 24, 2024
Spike doesn't support configuring which privilege modes are available (yet).
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