RFR: JDK-8328272: [AIX] Use flag kind "diagnostic" for platform specific flags

[Joachim Kern]

On Mon, 18 Mar 2024 11:24:37 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Current platform implementation (globals_aix.hpp) uses regular product flags for almost everything.
>> Most platform specific flags were never intended for official support. They are only there to diagnose issues and find workarounds.
>> So flag kind "diagnostic" fits better for them.
>
> Please be aware that this can cause problems with customers who update the JVM to this patch and have one of these settings set in production. Their JVMs may not come up after this change.
> 
> To use diagnostic flags, you need to specify "-XX:UnlockDiagnosticVMOptions" to unlock them, otherwise the JVM will not come up. Which is the reason such changes usually need CSRs.
> 
> Up to you if you risk it, ultimately. But I think `MaxExpectedDataSegmentSize` is needed and pretty commonly used. I would leave that one a product flag.

@tstuefe I'm fine with keeping `MaxExpectedDataSegmentSize`
The one I'm most interested to get rid of is `Use64KPages`. And I think you too, because then we can eliminate the 4K Pages support.
What about making the remaining 4 flags diagnostic in jdk23 and removing at least `Use64KPages` in jdk24?

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PR Comment: https://git.openjdk.org/jdk/pull/18337#issuecomment-2003899961