static inline unsigned ccnt_read(void) {
        unsigned cc;
        __asm__ volatile ("mrc p15, 0, %0, c15, c12, 1" : "=r"(cc));
        return cc;
}
void init_counter() {
// sets the cycle counter to work in /64 mode
        __asm__ volatile ("mcr p15, 0, %0, c15, c12, 0" :: "r"(13));
}
// http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0301h/Bihcjbaa.html