Re: [dpdk-dev] [PATCH RFC v2 08/12] lib/librte_vhost: vhost-user support

["Xie, Huawei" <huawei.xie@intel.com>]

> > +{
> > +	struct virtio_net *dev = get_device(ctx);
> > +
> > +	/* We have to stop the queue (virtio) if it is running. */
> > +	if (dev->flags & VIRTIO_DEV_RUNNING)
> > +		notify_ops->destroy_device(dev);
> 
> I have an one concern about finalization of vrings.
> Can vhost-backend stop accessing RX/TX to the vring before replying to
> this message?
> 
> QEMU sends this message when virtio-net device is finalized by
> virtio-net driver on the guest.
> After finalization, memories used by the vring will be freed by
> virtio-net driver, because these memories are allocated by virtio-net
> driver.
> Because of this, I guess vhost-backend must stop accessing to vring
> before replying to this message.
> 
> I am not sure what is a good way to stop accessing.
> One idea is adding a condition checking when rte_vhost_dequeue_burst()
> and rte_vhost_enqueue_burst() is called.
> Anyway we probably need to wait for stopping access before replying.
> 
> Thanks,
> Tetsuya
> 
I think we have discussed the similar question.
It is actually the same issue whether the virtio APP in guest is crashed, or is finalized.
The virtio APP will only write to the STATUS register without waiting/syncing to vhost backend.
After that, not only the guest memory pointed by vring entry but also the vring itself isn't usable any more.
The memory for vring or pointed by vring entry might be used by other APPs.
This will crash guest(rather than the vhost, do you agree?).

If you mean this issue, I think we have no solution but one walk around: keep the huge page files of crashed app, and 
bind virtio to igb_uio and then delete the huge page files.

In our implementation, when vhost sends the message,  we will call the destroy_device provided by the vSwitch to ask the
vSwitch to stop processing the vring, but this willn't solve the issue I mention above, because the virtio APP in guest will n't 
wait us.

Could you explain a bit more? Is it the same issue?


-huawei