//来自虚拟机的请求
static void usbredir_class_initfn(ObjectClass *klass, void *data)
{
    USBDeviceClass *uc = USB_DEVICE_CLASS(klass);
    DeviceClass *dc = DEVICE_CLASS(klass);

    uc->realize        = usbredir_realize;
    uc->product_desc   = "USB Redirection Device";
    uc->unrealize      = usbredir_unrealize;
    uc->cancel_packet  = usbredir_cancel_packet;
    uc->handle_reset   = usbredir_handle_reset;
    uc->handle_data    = usbredir_handle_data;
    uc->handle_control = usbredir_handle_control;
    uc->flush_ep_queue = usbredir_flush_ep_queue;
    uc->ep_stopped     = usbredir_ep_stopped;
    uc->alloc_streams  = usbredir_alloc_streams;
    uc->free_streams   = usbredir_free_streams;
    dc->vmsd           = &usbredir_vmstate;
    dc->props          = usbredir_properties;
    set_bit(DEVICE_CATEGORY_MISC, dc->categories);
}

//来自客户端的请求
    /* Let the backend know we are ready */
    qemu_chr_fe_set_handlers(&dev->cs, usbredir_chardev_can_read,
                             usbredir_chardev_read, usbredir_chardev_event,
                             NULL, dev, NULL, true);

static void usbredir_chardev_read(void *opaque, const uint8_t *buf, int size)
{
    USBRedirDevice *dev = opaque;

    /* No recursion allowed! */
    assert(dev->read_buf == NULL);

    dev->read_buf = buf;
    dev->read_buf_size = size;

    usbredirparser_do_read(dev->parser);
    /* Send any acks, etc. which may be queued now */
    usbredirparser_do_write(dev->parser);
}
static void usbredir_chardev_event(void *opaque, int event)
{
    USBRedirDevice *dev = opaque;

    switch (event) {
    case CHR_EVENT_OPENED:
        DPRINTF("chardev open\n");
        /* Make sure any pending closes are handled (no-op if none pending) */
        usbredir_chardev_close_bh(dev);
        qemu_bh_cancel(dev->chardev_close_bh);
        usbredir_create_parser(dev);
        break;
    case CHR_EVENT_CLOSED:
        DPRINTF("chardev close\n");
        qemu_bh_schedule(dev->chardev_close_bh);
        break;
    }
}