The kernel doesn't save any of these registers on entry into the kernel (via an interrupt or system call) and doesn't restore them on exit. It also doesn't save and restore them on internal scheduling. If you compile the kernel with MMX or SSE<n> enabled the kernel will sooner or later globber some state and you get a panic. It would be nice if the kernel had some gcc flags or similar would actually prevent compilation if any unsafe operations would be emitted.
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