RFR: 8284196: RISC-V: Detect supported ISA extensions over cpuinfo [v2]

[Feilong Jiang]

On Tue, 7 Feb 2023 08:30:12 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> Feilong Jiang has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fix typo
>
> src/hotspot/cpu/riscv/vm_version_riscv.cpp line 186:
> 
>> 184:   }
>> 185: 
>> 186:   if (UseZba && !_cpu_features.ext_zba) {
> 
> We are missing where we are implicitly enabling `UseZ*` based on their corresponding values in `_cpu_feature.ext_z*`. I would assume it to be the main motivator for this change.

Unlike RVC, there is still no hardware that supports those extensions. I think we should do more testing and evaluation on real hardware before enabling them by default.

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PR: https://git.openjdk.org/jdk/pull/12343