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riscv / riscv-fast-interrupt

Proposal for a RISC-V Core-Local Interrupt Controller (CLIC)
https://jira.riscv.org/browse/RVG-63
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The smclicshv extension naming (i.e. sm*) suggests it only has M-mode features but it affects S-mode and has one S-mode CSR bit #421

Open james-ball-qualcomm opened 1 month ago

james-ball-qualcomm commented 1 month ago

The smclicshv extension naming (i.e. sm*) suggests it only has M-mode features. Does it have S-mode features too? I ask because the smclicshv chapter uses the 'x' prefix notation instead of 'm'. What gives?

jb-brelot-nxp commented 1 month ago

From my side this extension is affecting all mode supported by the CLIC.

If CLIC is supporting only M mode, then this extension will affect only M registers If CLIC is supporting M and S mode, then this extension will affect M and S mode.

As soon as you get Selective Hardware vectoring supported on M mode, the cost to support it for S is for free. So x prefix is the good one from my side.

Maybe I forget some aspects. We can discuss this tonight.

christian-herber-nxp commented 1 month ago

i think there is no precedent for an x prefix in an extension. the normal thing to do is to have a smclicshv and a ssclicshv. However, here we do not want them to be selectable individually. So if smclicshv and sslic are supported, ssclicshv is mandatory. Otherwise, ssclicshv is forbidden.

james-ball-qualcomm commented 1 month ago

Precedent in Priv ISA manual:

Chapter 5. "Smcsrind/Sscsrind" Indirect CSR Access, Version 1.0.0 5.1. Introduction Smcsrind/Sscsrind is an ISA extension that extends the indirect CSR access mechanism originally defined as part of the Smaia/Ssaia extensions, in order to make it available for use by other extensions without creating an unnecessary dependence on Smaia/Ssaia. This extension confers two benefits:

james-ball-qualcomm commented 1 month ago

Another:

Chapter 4. "Smstateen/Ssstateen" Extensions, Version 1.0.0