[crac] RFR: 8355974: [CRaC] Move CPUFeatures verification to the parent process of JVM [v5]

[Jan Kratochvil]

On Tue, 29 Apr 2025 17:43:12 GMT, Timofei Pushkin <tpushkin at openjdk.org> wrote:

>> No longer:
>> 
>> crac_engine.cpp:443:32: error: passing ‘const CracEngine’ as ‘this’ argument discards qualifiers [-fpermissive]
>>   443 |   switch (prepare_user_data_api()) {
>
> I actually was imagining `prepare_user_data_api` to be called in `crac.cpp` like with the other `prepare_*_api` methods, not inside these functions. And also when getting `ApiStatus::UNSUPPORTED` do something else instead of failing like now.
> 
> Sorry, I forgot to write that initially: API extensions, like user data, ideally should not be mandatory, i.e. we shouldn't fail if an engine does not support an extension. In the case of CPU features I think this can be done meaningfully: if user data is not supported and `-XX:CPUFeatures` was not set we can emit a warning (on checkpoint and on every subsequent restore, `-XX:CPUFeatures=ignore`/`IgnoreCPUFeatures` can be used to silence the warning) and try to restore without verifying the features.

Refactored in b0c8012b5173fd049844f0d4f33e2f58f10af96f but I am not sure it is as you intended.

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PR Review Comment: https://git.openjdk.org/crac/pull/227#discussion_r2068575308