> the Linux kernel for example doesn't use any floating point or vector instructions

it doesn't use them by default, but code that needs the FPU can call kernel_fpu_begin() to save those registers if needed. https://yarchive.net/comp/linux/kernel_fp.html One big user is the AMD graphics driver: https://lore.kernel.org/amd-gfx/ee26b06b-f1a0-5a8f-388c-13c1...