8382337: [CRaC] Add CPU features nicknames for microarch levels by antonvoznia · Pull Request #309 · openjdk/crac

antonvoznia · 2026-04-16T12:30:05Z

x86_64 defines microarchitecture levels v1-v4: https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels

Let’s allow users to set -XX:CPUFeatures=x86-64-vX in addition to the current options (generic, native, ignore and bitmask).

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JDK-8382337: [CRaC] Add CPU features nicknames for microarch levels (Enhancement - P4)

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bridgekeeper · 2026-04-16T12:31:35Z

👋 Welcome back antonvoznia! A progress list of the required criteria for merging this PR into crac will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2026-04-16T12:31:59Z

@antonvoznia This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8382337: [CRaC] Add CPU features nicknames for microarch levels

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mlbridge · 2026-04-16T12:38:48Z

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01: Full - Incremental (7e28cf08)
00: Full (724a63d6)

rvansa

I see that you've followed the information on wiki, but from a practical point of you I am not sure if the FPU is present on modern CPUs. I will check AWS instance types and report my findings.
Also, I am somewhat concerned that v1 is not strictly superset of generic; we shall check out the FLUSH flag. Maybe these two could be equivalent?

Please add a test for the parsing; you can extend CheckCPUFeaturesTest or create a standalone one.

rvansa · 2026-04-16T12:37:43Z

+#ifdef AMD64
+  const char *microarch_prefix="x86-64-v";
+  const size_t microarch_prefix_len = strlen(microarch_prefix);
+  if (strncmp(str, microarch_prefix, strlen(microarch_prefix)) == 0 &&


replace the strlen(...) in here

rvansa · 2026-04-16T12:39:51Z

 }

+#if defined(LINUX) && defined(AMD64)
+void VM_Version::set_microarch_features(const char microarch_level, VM_Version::VM_Features &features) {


Could this return VM_Features rather than just modify reference?

rvansa · 2026-04-16T13:52:19Z

    uint32_t value;
    struct {
-      uint32_t          : 4,
+      uint32_t fpu      : 1,


Could you add a link to the struct with some reference saying where the bit position comes from?

antonvoznia · 2026-04-16T15:30:23Z

@rvansa , Thank you for reviewing!

I see that you've followed the information on wiki, but from a practical point of you I am not sure if the FPU is present on modern CPUs. I will check AWS instance types and report my findings.

I verified on my x86 vm

$ cat /proc/cpuinfo | grep -i fpu
fpu             : yes
fpu_exception   : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr....

Also, I am somewhat concerned that v1 is not strictly superset of generic; we shall check out the FLUSH flag. Maybe these two could be equivalent?

I followed this pages
https://en.opensuse.org/X86-64_microarchitecture_levels
and
https://gitlab.com/x86-psABIs/x86-64-ABI ,
there is no FLUSH mentioned.
Do you think we need to check it out for some reason?

I also didn't include flags OSFXSR and SCE. To have them, we need priveledge access to registers CR4 and MRS
CR4
https://stackoverflow.com/a/10186910
MRS
https://wiki.osdev.org/Model_Specific_Registers

Please add a test for the parsing; you can extend CheckCPUFeaturesTest or create a standalone one.

I'll add this

rvansa · 2026-04-16T20:36:31Z

I have verified that all Intel/AMD CPUs used for AWS instance types have the FPU flag in lscpu, so that should be good.

@jankratochvil Could you comment why is the FLUSH (clflush) "required by OpenJDK"?

rvansa · 2026-04-16T20:39:21Z

Actually the reason is documented in VM_Version::supports_clflush() - let's just add it to the required set, please.

jankratochvil · 2026-04-17T01:24:20Z

  union ExtCpuid1Edx {
    uint32_t value;
    struct {
-      uint32_t           : 22,


fpu should not be added anywhere. It is only set, but nothing ever reads or queries it. The only code that queries it is already disabled.

// FPU is always present on i686+: _features.supports_feature(CPU_FPU) &&

jankratochvil · 2026-04-17T01:32:44Z

There is an existing message in the code:

      log_error(crac)("Restore failed due to incompatible or missing CPU features, try using -XX:CPUFeatures=%s on checkpoint.", image_features.print_numbers());

The question is whether it should not say (which would make it a bit long and complicated suggestion for the user):

      log_error(crac)("Restore failed due to incompatible or missing CPU features, try using -XX:CPUFeatures=x86-64-v%c or -XX:CPUFeatures=%s on checkpoint.", image_features.print_level_TBD(), image_features.print_numbers());

or even just (which would make the performance sub-optimal due to some CPU features possibly missing):

      log_error(crac)("Restore failed due to incompatible or missing CPU features, try using -XX:CPUFeatures=x86-64-v%c on checkpoint.", image_features.print_level_TBD());

As otherwise I have some doubts anyone will ever find the x86-64-v%c possibility exists (nobody reads the docs) and which level is his/her CPU.

jankratochvil · 2026-04-17T01:50:24Z

+      features.set_feature(CPU_MMX);
+      features.set_feature(CPU_SSE);
+      features.set_feature(CPU_SSE2);
+      break;


Suggested change

break;

// The following options are all in /proc/cpuinfo of one of the first 64-bit CPUs - Atom D2700 (and Opteron 1352): https://superuser.com/q/1572306/1015048

features.set_feature(CPU_FLUSH);

features.set_feature(CPU_TSC);

break;

I do not know why they are not in the x86_64-v1 official set. I have already listed them for "generic" as they are required or useful for OpenJDK.

antonvoznia added 7 commits April 16, 2026 12:06

Initial implementation

c07c5b9

Fix static

03c0b68

Add 3-4 levels

58c866f

Fix typo

f3871bc

Fail with a message. Adding FPU in usupported by glibc

5f619b1

fix error message

de66395

Removed comments. Improved error message.

724a63d

openjdk bot added ready Pull request is ready to be integrated rfr Pull request is ready for review labels Apr 16, 2026

rvansa reviewed Apr 16, 2026

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8382337: [CRaC] Add CPU features nicknames for microarch levels#309

8382337: [CRaC] Add CPU features nicknames for microarch levels#309
antonvoznia wants to merge 8 commits intoopenjdk:cracfrom
antonvoznia:JDK-8382337-features-microarch-levels-impl

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-      break;
+      // The following options are all in /proc/cpuinfo of one of the first 64-bit CPUs - Atom D2700 (and Opteron 1352): https://superuser.com/q/1572306/1015048
+      features.set_feature(CPU_FLUSH);
+      features.set_feature(CPU_TSC);
+      break;

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