[crac] RFR: 8355974: [CRaC] Move CPUFeatures verification to the parent process of JVM [v11]

[Timofei Pushkin]

On Tue, 13 May 2025 18:19:50 GMT, Timofei Pushkin <tpushkin at openjdk.org> wrote:

>> src/hotspot/share/runtime/crac.cpp line 133:
>> 
>>> 131:         "with the selected CRaC engine");
>>> 132:       break;
>>> 133:   }
>> 
>> A UX improvement: if the user asks to ignore CPU features or the platform doesn't support them the user should not see warnings/errors about the engine not being able to store them.
>> 
>> Suggestion:
>> 
>>   if (!VM_Version::ignore_cpu_features()) {
>>     VM_Version::CPUFeaturesBinary data;
>>     if (VM_Version::cpu_features_binary(&data)) {
>>       switch (_engine->prepare_user_data_api()) {
>>         case CracEngine::ApiStatus::OK:
>>           if (!_engine->cpufeatures_store(&data)) {
>>             return JVM_CHECKPOINT_ERROR;
>>           }
>>           break;
>>         case CracEngine::ApiStatus::ERR:
>>           return JVM_CHECKPOINT_ERROR;
>>         case CracEngine::ApiStatus::UNSUPPORTED:
>>           log_warning(crac)("Cannot store CPUFeatures for checkpoint "
>>             "with the selected CRaC engine");
>>           break;
>>       }
>>     }
>>   }
>
> I realized the above won't work with the current restore code: if we won't record the features because the user asked us to ignore them but the platform and engine supports them then the restore code will error because the features are not present.
> 
> But without that the UX is not great and I don't really understand how `CPUFeatures=ignore` is supposed to work (it is not restore-settable but it is not used on checkpoint).
> 
> So maybe it's better to change the restore code to warn instead of erroring when the features are not present (and the warning should ideally be silenceable by `CPUFeatures=ignore` or `IgnoreCPUFeatures`).

This can be postponed until the `CPUFeatures=ignore`/`IgnoreCPUFeatures` cleanup.

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PR Review Comment: https://git.openjdk.org/crac/pull/227#discussion_r2087412077