Re: [Qemu-devel] [PATCH 1/3] virtio: introduce grab/release_ioeventfd to fix vhost

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 18, 2016 at 09:15:32AM +0100, Christian Borntraeger wrote:
> On 11/16/2016 07:05 PM, Paolo Bonzini wrote:
> > Following the recent refactoring of virtio notifiers [1], more specifically
> > the patch ed08a2a0b ("virtio: use virtio_bus_set_host_notifier to
> > start/stop ioeventfd") that uses virtio_bus_set_host_notifier [2]
> > by default, core virtio code requires 'ioeventfd_started' to be set
> > to true/false when the host notifiers are configured.
> > 
> > When vhost is stopped and started, however, there is a stop followed by
> > another start. Since ioeventfd_started was never set to true, the 'stop'
> > operation triggered by virtio_bus_set_host_notifier() will not result
> > in a call to virtio_pci_ioeventfd_assign(assign=false). This leaves
> > the memory regions with stale notifiers and results on the next start
> > triggering the following assertion:
> > 
> >   kvm_mem_ioeventfd_add: error adding ioeventfd: File exists
> >   Aborted
> > 
> > This patch reintroduces (hopefully in a cleaner way) the concept
> > that was present with ioeventfd_disabled before the refactoring.
> > When ioeventfd_grabbed>0, ioeventfd_started tracks whether ioeventfd
> > should be enabled or not, but ioeventfd is actually not started at
> > all until vhost releases the host notifiers.
> > 
> > [1] http://lists.nongnu.org/archive/html/qemu-devel/2016-10/msg07748.html
> > [2] http://lists.nongnu.org/archive/html/qemu-devel/2016-10/msg07760.html
> > 
> > Reported-by: Felipe Franciosi <felipe@nutanix.com>
> > Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
> > Reported-by: Alex Williamson <alex.williamson@redhat.com>
> > Fixes: ed08a2a0b ("virtio: use virtio_bus_set_host_notifier to start/stop ioeventfd")
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > Message-Id: <20161111192855.26350-1-pbonzini@redhat.com>
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >         v1->v2: more comments [Cornelia]
> 
> 
> As this seems to fix a functional issues, is there any chance to apply this
> patch now and not wait for the discussion about patch 2 and 3 to calm down?

It's in my tree, will be in the next pull.