Fix vec_caps to test for OS support too (on x64)

[ldematte]

On x64, we are testing if we support vector capabilities (1 = "basic" = AVX2, 2 = "advanced" = AVX-512) in order to enable and choose a native implementation for some vector functions, using CPUID.

However, under some circumstances, this is not sufficient: the OS on which we are running also needs to support AVX/AVX2 etc; basically, it needs to acknowledge it knows about the additional register and that it is able to handle them e.g. in context switches. To do that we need to a) test if the CPU has xsave feature and b) use the xgetbv to test if the OS set it (declaring it supports AVX/AVX2/etc).

In most cases this is not needed, as all modern OSes do that, but for some virtualized situations (hypervisors, emulators, etc.) all the component along the chain must support it, and in some cases this is not a given.

This PR introduces a change to the x64 version of vec_caps to check for OS support too, and a warning on the Java side in case the CPU supports vector capabilities but those are not enabled at OS level.

Tested by passing noxsave to my linux box kernel boot options, and ensuring that the avx flags "disappear" from /proc/cpuinfo, and we fall back to the "no native vector" case.

Fixes #126809

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Hi @ldematte, I've created a changelog YAML for you.

[ldematte]

Tested locally:
Normally:

[2025-04-16T16:23:32,610][INFO ][o.e.n.j.JdkVectorLibrary ] [runTask-0] vec_caps=1

Booting kernel with noxsave
No avx or similar in /proc/cpuinfo:

flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb invpcid_single pti ssbd ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 smep bmi2 erms invpcid rdseed adx smap clflushopt intel_pt dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp md_clear flush_l1d arch_capabilities

ES logs:

[2025-04-16T16:50:58,810][INFO ][o.e.n.j.JdkVectorLibrary ] [runTask-0] vec_caps=-1
[2025-04-16T16:50:58,811][WARN ][o.e.n.j.JdkVectorLibrary ] [runTask-0] Your CPU supports vector capabilities, but they are disabled at OS level. For optimal performance, enable them in your OS/Hypervisor/VM/container

[ChrisHegarty]

Thanks for the fix @ldematte. LGTM

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.18
✅	8.x
✅	9.0
✅	8.17