The OS can just turn off any instruction extensions by setting the XCR0 register to whatever it wants. Unsupported instructions will then cause general protection faults (SIGSEGV on Linux). Whether that's what the OS wants to do is a totally separate question. It would be kinda funny if this thing lead to the ability of a process to request a certain XCR0 that the kernel would then serve, because I've been asking for that feature for unrelated reasons for quite some time and didn't get the warmest response to it on the mailing lists.