qemu_clock_get_ns函數分析

1.函數總體 

int64_t qemu_clock_get_ns(QEMUClockType type)
{
    int64_t now, last;
    QEMUClock *clock = qemu_clock_ptr(type);

    switch (type) {
    case QEMU_CLOCK_REALTIME:
        return get_clock();
    default:
    case QEMU_CLOCK_VIRTUAL:   //由於在openpic中創建timer時用的是虛擬時鐘，因此只分析這種case
        if (use_icount) {
            return cpu_get_icount();
        } else {
            return cpu_get_clock();
        }
    case QEMU_CLOCK_HOST:
        now = REPLAY_CLOCK(REPLAY_CLOCK_HOST, get_clock_realtime());
        last = clock->last;
        clock->last = now;
        if (now < last || now > (last + get_max_clock_jump())) {
            notifier_list_notify(&clock->reset_notifiers, &now);
        }
        return now;
    case QEMU_CLOCK_VIRTUAL_RT:
        return REPLAY_CLOCK(REPLAY_CLOCK_VIRTUAL_RT, cpu_get_clock());
    }
}

2.cpu_get_icount函數分析

/* Return the virtual CPU time, based on the instruction counter.  */
int64_t cpu_get_icount(void)
{
    int64_t icount;
    unsigned start;

    do {
        start = seqlock_read_begin(&timers_state.vm_clock_seqlock);
        icount = cpu_get_icount_locked();
    } while (seqlock_read_retry(&timers_state.vm_clock_seqlock, start));

    return icount;
}

3. cpu_get_clock函數分析

int64_t cpu_get_clock(void)
{
    return get_clock_realtime();
}

/* get host real time in nanosecond */
static inline int64_t get_clock_realtime(void)
{
    struct timeval tv;

    gettimeofday(&tv, NULL); //通過該函數獲取當前的精確時間
    /*因爲1納秒=10^-9s，1微妙=10^-6秒，最終返回的結果是ns，因此對s*10^9，對us*10^3/
    return tv.tv_sec * 1000000000LL + (tv.tv_usec * 1000);
}