Re: [PATCH v2 5/5] vhost: add device started check in migration set log

[Jason Wang <jasowang@redhat.com>]

On 2020/4/30 下午9:36, Dima Stepanov wrote:
> If vhost-user daemon is used as a backend for the vhost device, then we
> should consider a possibility of disconnect at any moment. If such
> disconnect happened in the vhost_migration_log() routine the vhost
> device structure will be clean up.
> At the start of the vhost_migration_log() function there is a check:
>    if (!dev->started) {
>        dev->log_enabled = enable;
>        return 0;
>    }
> To be consistent with this check add the same check after calling the
> vhost_dev_set_log() routine. This in general help not to break a
> migration due the assert() message. But it looks like that this code
> should be revised to handle these errors more carefully.
>
> In case of vhost-user device backend the fail paths should consider the
> state of the device. In this case we should skip some function calls
> during rollback on the error paths, so not to get the NULL dereference
> errors.
>
> Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
> ---
>   hw/virtio/vhost.c | 39 +++++++++++++++++++++++++++++++++++----
>   1 file changed, 35 insertions(+), 4 deletions(-)
>
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 3ee50c4..d5ab96d 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -787,6 +787,17 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
>   static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
>   {
>       int r, i, idx;
> +
> +    if (!dev->started) {
> +        /*
> +         * If vhost-user daemon is used as a backend for the
> +         * device and the connection is broken, then the vhost_dev
> +         * structure will be reset all its values to 0.
> +         * Add additional check for the device state.
> +         */
> +        return -1;
> +    }
> +
>       r = vhost_dev_set_features(dev, enable_log);
>       if (r < 0) {
>           goto err_features;
> @@ -801,12 +812,19 @@ static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
>       }
>       return 0;
>   err_vq:
> -    for (; i >= 0; --i) {
> +    /*
> +     * Disconnect with the vhost-user daemon can lead to the
> +     * vhost_dev_cleanup() call which will clean up vhost_dev
> +     * structure.
> +     */
> +    for (; dev->started && (i >= 0); --i) {
>           idx = dev->vhost_ops->vhost_get_vq_index(


Why need the check of dev->started here, can started be modified outside 
mainloop? If yes, I don't get the check of !dev->started in the 
beginning of this function.


> dev, dev->vq_index + i);
>           vhost_virtqueue_set_addr(dev, dev->vqs + i, idx,
>                                    dev->log_enabled);
>       }
> -    vhost_dev_set_features(dev, dev->log_enabled);
> +    if (dev->started) {
> +        vhost_dev_set_features(dev, dev->log_enabled);
> +    }
>   err_features:
>       return r;
>   }
> @@ -832,7 +850,15 @@ static int vhost_migration_log(MemoryListener *listener, int enable)
>       } else {
>           vhost_dev_log_resize(dev, vhost_get_log_size(dev));
>           r = vhost_dev_set_log(dev, true);
> -        if (r < 0) {
> +        /*
> +         * The dev log resize can fail, because of disconnect
> +         * with the vhost-user-blk daemon. Check the device
> +         * state before calling the vhost_dev_set_log()
> +         * function.
> +         * Don't return error if device isn't started to be
> +         * consistent with the check above.
> +         */
> +        if (dev->started && r < 0) {
>               return r;
>           }
>       }
> @@ -1739,7 +1765,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
>   fail_log:
>       vhost_log_put(hdev, false);
>   fail_vq:
> -    while (--i >= 0) {
> +    /*
> +     * Disconnect with the vhost-user daemon can lead to the
> +     * vhost_dev_cleanup() call which will clean up vhost_dev
> +     * structure.
> +     */
> +    while ((--i >= 0) && (hdev->started)) {
>           vhost_virtqueue_stop(hdev,
>                                vdev,
>                                hdev->vqs + i,


This should be a separate patch.

Thanks