[hansendc]

The processor has XSAVE (the mechanism that we use to save/restore FPU state and more these days) optimizations internal to the processor that keep it from having to fully reload the FPU state. OSes like Linux have not been doing lazy FPU switching on processors with these optimizations for a long time.

See information about XSAVEOPT and the "Init and Modified Optimizations" in the SDM: intel.com/sdm .

As @luto said above, recent versions of Linux ripped out the lazy handling entirely.