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

[Fei Yang]

On Mon, 22 Jan 2024 15:10:53 GMT, Hamlin Li <mli at openjdk.org> wrote:

> Hi,
> Can you help to review this simple patch to add hw probe flags?
> Thanks!
> 
> Per https://github.com/torvalds/linux/blob/master/arch/riscv/include/uapi/asm/hwprobe.h, several extensions were already added in kernel code, we have several intrinsics or funtionalities depends on these extension, it's ready to add these hw probe flags already used in jdk into jdk code.

PS: I just realized that this will auto-enable experimental RV-specific JVM options like `UseZtso`, `UseZacas`, etc. when the features are available, something which I think we shouldn't do for now. Those experimental options are not fully tested on real hardwares yet. Maybe remove `UPDATE_DEFAULT(UseZtso)`, `UPDATE_DEFAULT(UseZtso)` from `ext_Ztso`, `ext_Zacas` declarations at the same time? Or postpone this change for some time after we have the hardware?

-------------

PR Comment: https://git.openjdk.org/jdk/pull/17519#issuecomment-1905776489