Subject: Re: Query: Is it possible to lose interrupts between vhost and virtio_net during migration?

From: Jason Wang <[email-masked]>
Date: Fri, 01 Aug 2014 18:47:59 +0800

On 07/31/2014 10:37 PM, Michael S. Tsirkin wrote:
> On Thu, Jul 31, 2014 at 04:31:00PM +0200, Michael S. Tsirkin wrote:
>> > On Thu, Jul 31, 2014 at 07:47:24PM +0800, Zhangjie (HZ) wrote:
>>> > > [The test scenario]:
>>> > > 
>>> > > Doing migration between two Hosts roundly(A->B, B->A) ,after about 20 times, network of the VM is unreachable.
>>> > > There are other 20 VMs in each Host, and they send ipv4 or ipv6 and multicast packets to each other.
>>> > > Sometimes the CPU idle of the Host maybe 0;
>>> > > 
>>> > > [Problem description]:
>>> > > 
>>> > > I wonder if it was interrupts missing that cause the network unreachable.
>>> > > In the migration process of kvm, source end should suspend, which include steps as follows:
>>> > > 1.	do_vm_stop->pause_all_vcpus
>>> > > 2.	vm_state_notify-> vhost_net_stop->set_guest_notifiers->kvm_virtio_pci_vq_vector_release
>>> > > 3.	vm_state_notify-> vhost_net_stop-> vhost_net_stop_one->OST_NET_SET_BACKEND-> vhost_net_flush_vq-> vhost_work_flush
>>> > > This may cause interrupts missing. Supose the scene that, virtqueue_notify() is called in virtio_net,
>>> > > then the VM is paused. And, just before the portiowrite being handled, eventfd of kvm is released.
>>> > > Then, vhost could not sense the notify, and the tx notify is lost.
>>> > > On the other side, if eventfd of kvm is released just after vhost_notify(), and before eventfd_signal(), then rx signal by vhost is lost.
>> > 
>> > Could be a bug in userspace: should should cleanups notifiers
>> > after it stops vhost.
>> > 
>> > Could you please send this to appropriate mailing lists?
>> > I have a policy against off-list discussions.
> Also, Jason, could you take a look please?
> Looks like your patch a9f98bb5ebe6fb1869321dcc58e72041ae626ad8
> changed the order of stopping the device.
> Previously vhost_dev_stop would disable backend and only afterwards,
> unset guest notifiers.  You now unset guest notifiers while vhost is still
> active. Looks like this can lose events?

Not sure it will really cause the issue. Since during guest notifier
deassign in virtio_queue_set_guest_notifier_fd_handler() it will test
the notifier and trigger callback if set. Looks like this can guarantee
the interrupt was not lost.