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

[Jan Kratochvil]

On Tue, 29 Apr 2025 08:35:52 GMT, Timofei Pushkin <tpushkin at openjdk.org> wrote:

>> There was originally a mistake:
>> - restoring JVM did restore the image
>> - the restored JVM started checking whether CPU Features of the new host >= CPU Features of the checkpoint host
>> 
>> That is difficult as glibc is already configured (IFUNC) in the image for the checkpoint host and calling any such glibc functions in the restored image will crash (as the advanced instructions from misconfigured IFUNC are not available). Some glibc functions had to be reimplemented in a dummy way inside JVM due to this misdesign.
>> 
>> This patch changes it to:
>> - restoring JVM checks `cpufeatures` user data in the image against current CPU Features
>> - the restored JVM is started only if the CPU Features are satisfied, restored JVM no longer has to verify anything
>> 
>> The patch is a bit of a kitchen sink, there are various improvements of the CPU Features code.
>
> src/hotspot/share/runtime/crac_engine.hpp line 70:
> 
>> 68: 
>> 69:   bool cpufeatures_store();
>> 70:   bool cpufeatures_restore();
> 
> These can be marked `const`

No longer:

crac_engine.cpp:443:32: error: passing ‘const CracEngine’ as ‘this’ argument discards qualifiers [-fpermissive]
  443 |   switch (prepare_user_data_api()) {

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PR Review Comment: https://git.openjdk.org/crac/pull/227#discussion_r2066792194