tcler's blog --- 其实我是一个程序员
Show me your flowcharts and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won’t usually need your flowcharts; they’ll be obvious.

jiyin@max395:~/ws/tools/qemu-master$ grep -i "display output is not active" -r .
./ui/console.c:    static const char placeholder_msg[] = "Display output is not active.";
jiyin@max395:~/ws/tools/qemu-master$ vim ui/console.c

void qemu_console_set_surface(QemuConsole *con,
                             DisplaySurface *surface)
{
    static const char placeholder_msg[] = "Display output is not active.";
    DisplayState *s = con->ds;
    DisplaySurface *old_surface = con->surface;
    DisplaySurface *new_surface = surface;
    DisplayChangeListener *dcl;
    int width;
    int height;

    if (!surface) {
        if (old_surface) {
            width = surface_width(old_surface);
            height = surface_height(old_surface);
        } else {
            width = 640;
            height = 480;
        }

        new_surface = qemu_create_placeholder_surface(width, height, placeholder_msg);
    }

    assert(old_surface != new_surface);

    con->scanout.kind = SCANOUT_SURFACE;
    con->surface = new_surface;
    dpy_gfx_create_texture(con, new_surface);
    QLIST_FOREACH(dcl, &s->listeners, next) {
        if (con != dcl->con) {
            continue;
        }
        displaychangelistener_gfx_switch(dcl, new_surface, surface ? FALSE : TRUE);
    }
    dpy_gfx_destroy_texture(con, old_surface);
    qemu_free_displaysurface(old_surface);
}

jiyin@max395:~/ws/tools/qemu-master$ grep qemu_console_set_surface.*NULL -r .
./hw/display/vhost-user-gpu.c:            qemu_console_set_surface(con, NULL);
./hw/display/virtio-gpu-rutabaga.c:        qemu_console_set_surface(scanout->con, NULL);
./hw/display/virtio-gpu-rutabaga.c:    qemu_console_set_surface(scanout->con, NULL);
./hw/display/virtio-gpu-virgl.c:        qemu_console_set_surface(g->parent_obj.scanout[ss.scanout_id].con, NULL);
./hw/display/virtio-gpu-virgl.c:        qemu_console_set_surface(g->parent_obj.scanout[i].con, NULL);
./hw/display/virtio-gpu.c:    qemu_console_set_surface(scanout->con, NULL);
./hw/display/virtio-gpu.c:        qemu_console_set_surface(g->parent_obj.scanout[i].con, NULL);
jiyin@max395:~/ws/tools/qemu-master$ vim hw/display/virtio-gpu.c

static void virtio_gpu_reset_bh(void *opaque)
{
    VirtIOGPU *g = VIRTIO_GPU(opaque);
    VirtIOGPUClass *vgc = VIRTIO_GPU_GET_CLASS(g);
    struct virtio_gpu_simple_resource *res, *tmp;
    uint32_t resource_id;
    Error *local_err = NULL;
    int i = 0;

    QTAILQ_FOREACH_SAFE(res, &g->reslist, next, tmp) {
        resource_id = res->resource_id;
        vgc->resource_destroy(g, res, &local_err);
        if (local_err) {
            error_append_hint(&local_err, "%s: %s resource_destroy"
                              "for resource_id = %"PRIu32" failed.\n",
                              __func__, object_get_typename(OBJECT(g)),
                              resource_id);
            /* error_report_err frees the error object for us */
            error_report_err(local_err);
            local_err = NULL;
        }
    }

    for (i = 0; i < g->parent_obj.conf.max_outputs; i++) {
        qemu_console_set_surface(g->parent_obj.scanout[i].con, NULL);  //<--- 大概就是这里了
    }

    g->reset_finished = true;
    qemu_cond_signal(&g->reset_cond);
}

OS 加载 virtio-gpu 驱动
    ↓
驱动通过 PCIe 配置空间写命令寄存器 (触发设备复位)
    ↓
QEMU PCI 模拟层捕获写操作
    ↓
调用 VirtIO-PCI 层的 virtio_pci_reset() 或类似函数
    ↓
virtio_gpu_reset()  [hw/display/virtio-gpu.c]
    ↓
调度下半部 virtio_gpu_reset_bh()
    ↓
对每个 scanout 调用 qemu_console_set_surface(con, NULL)
    ↓
qemu_create_placeholder_surface() 生成 "Display output is not active."
    ↓
VNC 客户端显示这条消息

QemuConsole *qemu_graphic_console_create(DeviceState *dev, uint32_t head,
                                         const GraphicHwOps *hw_ops,
                                         void *opaque)
{
    static const char noinit[] =
        "Guest has not initialized the display (yet).";
    int width = 640;
    int height = 480;
    QemuConsole *s;
    DisplaySurface *surface;

    s = qemu_graphic_console_lookup_unused();
    if (s) {
        trace_console_gfx_reuse(s->index);
        width = qemu_console_get_width(s, 0);
        height = qemu_console_get_height(s, 0);
    } else {
        trace_console_gfx_new();
        s = (QemuConsole *)object_new(TYPE_QEMU_GRAPHIC_CONSOLE);
    }
    QEMU_GRAPHIC_CONSOLE(s)->head = head;
    qemu_graphic_console_set_hwops(s, hw_ops, opaque);
    if (dev) {
        object_property_set_link(OBJECT(s), "device", OBJECT(dev),
                                 &error_abort);
    }

    surface = qemu_create_placeholder_surface(width, height, noinit);
    qemu_console_set_surface(s, surface);
    s->gl_unblock_timer = timer_new_ms(QEMU_CLOCK_REALTIME,
                                       console_hw_gl_unblock_timer, s);
    return s;
}