RFR: 8324304: RISC-V: add hw probe flags

[Fei Yang]

On Tue, 23 Jan 2024 11:57:39 GMT, Hamlin Li <mli at openjdk.org> wrote:

> I just tested on `Lichee Pi 4A`, seems the default behaviour is to disable them.
> 
> ```
> echo "enable explicitly ......"
> TEST_OPTIONS="-XX:+UseZicboz -XX:+UseZtso -XX:+UseZacas -XX:+UseZfh"
> $J_H/bin/java -XX:+PrintFlagsFinal -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions ${TEST_OPTIONS} -version | grep -e UseZicboz -e UseZtso -e UseZacas -e UseZfh
> 
> echo "by default ......"
> TEST_OPTIONS=""
> $J_H/bin/java -XX:+PrintFlagsFinal -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions ${TEST_OPTIONS} -version | grep -e UseZicboz -e UseZtso -e UseZacas -e UseZfh
> ```
> 
> output is as below:
> 
> ```
> enable explicitly ......
>      bool UseZacas                                 = true                            {ARCH experimental} {command line}
>      bool UseZfh                                   = true                                 {ARCH product} {command line}
>      bool UseZicboz                                = true                            {ARCH experimental} {command line}
>      bool UseZtso                                  = true                            {ARCH experimental} {command line}
> by default ......
>      bool UseZacas                                 = false                           {ARCH experimental} {default}
>      bool UseZfh                                   = false                                {ARCH product} {default}
>      bool UseZicboz                                = false                           {ARCH experimental} {default}
>      bool UseZtso                                  = false                           {ARCH experimental} {default}
> ```

Say, when you are running some latest kernel with the hwprobe capability on some new RISC-V hardwares with those extensions. 
Then these JVM options will be auto-enabled, right? But I think we need some time to fully test them (especially, `-XX:+UseZtso` and `-XX:+UseZacas`) before that happens. AFAIK, Lichee Pi 4A is not equipped with those extensions and is running some older kernel.

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PR Comment: https://git.openjdk.org/jdk/pull/17519#issuecomment-1907173031