[crac] RFR: Support -XX:CPUFeatures=ignore during -XX:CRaCRestoreFrom

[Anton Kozlov]

On Wed, 9 Aug 2023 15:08:56 GMT, Radim Vansa <rvansa at openjdk.org> wrote:

>> Support -XX:CPUFeatures=ignore during -XX:CRaCRestoreFrom
>
> src/hotspot/cpu/x86/globals_x86.hpp line 229:
> 
>> 227:       "get an error during -XX:CRaCRestoreFrom on a different machine; "    \
>> 228:       "-XX:CPUFeatures=native is the default; "                             \
>> 229:       "-XX:CPUFeatures=ignore will disable the CPU features check; "        \
> 
> What happens when you set `CPUFeatures=ignore` on checkpoint?

That should make to use the current CPU features, the option was supported, apparently just not documented.

> src/hotspot/cpu/x86/vm_version_x86.cpp line 2569:
> 
>> 2567:       tty->print_raw(part2, sizeof(part2) - 1);
>> 2568:       tty->cr();
>> 2569:       vm_exit_during_initialization();
> 
> Will this work when you set `CPUFeatures=generic` before checkpoint and don't set anything during restore? The option will keep its value and this might trigger the error.
> If you don't want to add some extra tests you might piggyback in `VMOptionsTest`.

Agree. And we are unable to handle e.g. `generic` specified for restore, although it's documented. 

Having dedicated IgnoreCPUFeatures would simplify the implementation a lot, and we'd also could make it Experimental, as it should be.

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PR Review Comment: https://git.openjdk.org/crac/pull/100#discussion_r1289132670
PR Review Comment: https://git.openjdk.org/crac/pull/100#discussion_r1289140354