FW: question about JDK-8172231: SPARC CPU features detection

Ilarion Nakonechnyy ilarion at azul.com
Mon Apr 19 19:21:28 UTC 2021

Dear sirs, good day.
My name is Ilarion, I’m new contributor for OpenJDK project.

I have some questions about implementation of fix for bug JDK-8172231: SPARC CPU features detection, probably somebody may shed some lights on it.

I’m doing my exercises on JDK 11 and noticed that on exact SPARC CPU block zeroing detection feature determines wrong.
Having system with SPARC64-X CPU, java fails with segmentation violation error, on the very beginning of the java run.
My investigations lead me to MacroAssembler::bis_zeroing() routine.  Where begin and end of memory area is cleared by stx instruction, and the rest – via stxa(G0, to, G0, Assembler::ASI_ST_BLKINIT_PRIMARY), that supposed to zeroing memory, but it doesn’t.
Turning off block zeroing feature via flag -XX:-UseBlockZeroing helps to avoid the crash.

Decision if CPU has the CPU_blk_zeroing_msk feature based on determining another cpu flag,  ISA_ima_msk

+    // Niagara Core S3 supports fast RDPC and block zeroing.

+    if (has_ima()) {

+      synthetic |= (CPU_fast_rdpc_msk | CPU_blk_zeroing_msk);

+    }

But there is also mentioned special flag, that corresponds to Block initialization feature: ISA_blk_init_msk. That actually isn’t checked at all. ( Other than in reporting )
I looked in JDK 8, and  figured out, that block zeroing capacity there was decided not  only by ISA flags, but with referring on CPU model also:

-  // On T4 and newer Sparc BIS to the beginning of cache line always zeros it.

-  static bool has_block_zeroing()       { return has_blk_init() && is_T4(); }

JDK 8 works well for my case, because BIS instruction isn’t used
Despite ISA flags that is returned by getisax reports that block initialization is supported ( AV_SPARC_ASI_BLK_INIT 0x0080 )

[0.172s][info][os,cpu  ] getisax(2) returned 1 words:

[0.172s][info][os,cpu  ]     word 0: 0xc001b1f0

So, as I can see, the CPU version ( “cpu brand”  in jdk 8 ) has main role in detecting CPU block zeroing feature.
Do you have some more information, probably any doc, where noted why CPU version plays such a role in feature detecting for JVM? In CPU specifications I didn’t found any clues about this.
Thank you in advance.

More information about the jdk-updates-dev mailing list