Well, except that in kernel you often can't use even many CPU features either in practice. At least if you use SSE2/FP/etc., you better ensure those FPU regist

Well, except that in kernel you often can't use even many CPU features either in practice. At least if you use SSE2/FP/etc., you better ensure those FPU registers are saved and restored. But probably you don't want to do that in a IRQ handler! Save only SSE2 registers on an AVX system (higher 128 bits will be zeroed for currently executing usermode thread!) and receive "interesting" bug reports from the end users. If you refer to a vm page that's not present... well, bad things might happen. Etc.