RFR: 8330171: Lazy W^X switch implementation

[Dean Long]

On Fri, 12 Apr 2024 14:40:05 GMT, Sergey Nazarkin <snazarki at openjdk.org> wrote:

> An alternative for preemptively switching the W^X thread mode on macOS with an AArch64 CPU. This implementation triggers the switch in response to the SIGBUS signal if the *si_addr* belongs to the CodeCache area. With this approach, it is now feasible to eliminate all WX guards and avoid potentially costly operations. However, no significant improvement or degradation in performance has been observed.  Additionally, considering the issue with AsyncGetCallTrace, the patched JVM has been successfully operated with [asgct_bottom](https://github.com/parttimenerd/asgct_bottom) and [async-profiler](https://github.com/async-profiler/async-profiler). 
> 
> Additional testing:
> - [x] MacOS AArch64 server fastdebug *gtets*
> - [ ] MacOS AArch64 server fastdebug *jtreg:hotspot:tier4*
> - [ ] Benchmarking
> 
> @apangin and @parttimenerd could you please check the patch on your scenarios??

I think there is a sweet-spot middle-ground between the two extremes: full-lazy, ideal for performance, and fine-grained execute-by-default, ideal for security.  I don't think we should change to full-lazy and remove all the guard rails at this time.  I am investigating execute-by-default, and it looks promising.

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

Changes requested by dlong (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18762#pullrequestreview-2051465621