[jdk19] RFR: 8289302: Restore CriticalJNINatives

[Robbin Ehn]

On Wed, 29 Jun 2022 12:10:35 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This change restores the code in the JVM that implemented the internal JavaCritical or CriticalJNINatives functionality.  Customers are using this feature and there's no replacement yet for it.
> This change is a backout and merge of jDK-8258192: Obsolete the CriticalJNINatives flag.
> This change also adds the deprecation message that was missed, which I checked is okay with the customer.
> 
> Tested with tiers1-4.

I'm not convinced.

The example provided for a short native call that is a lot slower is:
clock_gettime(CLOCK_REALTIME)

This is vDSO syscall, meaning it is just reading mapped memory.
If performance was essential why is the user not mapping the vDSO memory directly into Java, thus plainly reading without even needing to do a native method call?

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PR: https://git.openjdk.org/jdk19/pull/90