Static BSP_GS_INITIALIZED 

static BSP_GS_INITIALIZED: AtomicBool

Set to true once the BSP has called init_gs_base.

Used by current_cpu_index() to skip the serialising rdmsr null-guard on every hot-path call once GS is permanently initialised.

Invariant: APs always call init_gs_base before the first call to current_cpu_index() on that AP (see smp_main in smp.rs). Once BSP_GS_INITIALIZED is true, any CPU that could possibly call current_cpu_index() must already have a valid GS base, so the direct gs:[0] read on the fast path is safe.