When an interface receives a packet, it sends an interrupt. Naturally, this causes its interrupt handler to execute. For example, here is what executes in em(4):

static void
em_msix_rx(void *arg)
{
        struct rx_ring *rxr = arg;
        struct adapter *adapter = rxr->adapter;
        bool more;

        ++rxr->rx_irq;

        more = em_rxeof(rxr, ...