## What happened
Elasticsearch 8.17.4 in Docker (OpenJDK 23, Ubuntu 20.04 container on rocky-linux host) crashed with a SIGILL in the native `libvec.so` library whenever a kNN search ran on a dense_vector field. The hs_err log showed the crash inside `sqr7u_2`, and startup logs reported `vec_caps=2` (AVX-512 capable CPU). Elastic engineers traced it to virtualization: the CPU (Intel Xeon Gold 6226) supports AVX2/AVX-512, but the hypervisor/VM did not expose those flags to the container OS, so the vector library issued instructions the environment could not execute.
## The fix that worked
Check that AVX2/AVX-512 is actually exposed at every layer: bare metal, hypervisor/VM, and container (`lscpu` flags inside the container must show avx/avx2/avx512). If your hypervisor hides those flags, either enable them in the hypervisor/VM config or start Elasticsearch with the native vector library disabled: `-Dorg.elasticsearch.nativeaccess.enableVectorLibrary=false`. An Elastic maintainer confirmed the fix direction is to make Elasticsearch fall back gracefully with a warning instead of crashing when the OS-level flags are missing.