RFR: 8318026: jcmd should provide access to low-level JVM debug information

[Serguei Spitsyn]

On Wed, 31 Jan 2024 14:22:44 GMT, Kevin Walls <kevinw at openjdk.org> wrote:

> Introduce the jcmd "VM.debug" to implement access to a useful set of the established debug.cpp utilities, with "jcmd PID VM.debug subcommand ...".
> 
> Not recommended for live production use.  Calling these "debug" utilities, and not including them in the jcmd help output, is to remind us they are not general customer-facing tools.

Kevin, thank you for working on this! I've posted several comments/questions.

src/hotspot/share/services/diagnosticCommand.cpp line 1200:

> 1198: void VMDebugDCmd::find() {
> 1199:   if (!_arg1.has_value()) {
> 1200:     output()->print_cr("missing argument");

I'm thinking if it would be useful to tell what arguments are expected? This is for all cases where the `"missing argument"` message is returned.

src/hotspot/share/services/diagnosticCommand.cpp line 1245:

> 1243:     }
> 1244:   } else if (strcmp("find", _subcommand.value()) == 0) {
> 1245:     if (!UnlockDiagnosticVMOptions) {

Would it make sense to require enabling `UnlockDiagnosticVMOptions` for all sub-commands, so that it is clear this is not for live production use?

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

PR Review: https://git.openjdk.org/jdk/pull/17655#pullrequestreview-1902349676
PR Review Comment: https://git.openjdk.org/jdk/pull/17655#discussion_r1503520152
PR Review Comment: https://git.openjdk.org/jdk/pull/17655#discussion_r1503518811