No, it's on a per-function basis. On program startup it does the necessary checks (CPUID etc.) and sets up the function pointers appropriately (see the IFUNC mechanism in the linker).
That's OK for code e.g. you know it could benefit from SIMD usage. However you can not tag every function of user code for safe/unsafe mode. Also, optimizations would increase the mess (inlining, unrolling, etc.). Generated code would be a "Frankenstein".
