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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 31 Jul 2014 16:31:00 +0200

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.

-- 
MST