Re: [Qemu-devel] [PATCH 0/4] Tweaks around virtio-blk start/stop

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Wed, 16 Mar 2016 13:32:59 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> On 16/03/2016 13:22, Cornelia Huck wrote:
> >> > Yeah, it doesn't help that the functions are underdocumented (as in the
> >> > "assign" parameter above).
> > My understanding is:
> > 
> > - 'assign': set up a new notifier (true) or disable it (false)
> > - 'set_handler': use our handler (true) or have it handled elsewhere
> >   (false)
> 
> Right.  So if we're setting up a new notifier in
> virtio_queue_aio_set_host_notifier_handler, virtio_pci_stop_ioeventfd should

...do what? ;)

> 
> > > > I don't think the ->set_host_notifiers() api really allows for this.
> > > 
> > > I think it does, assign is the last argument to k->set_host_notifier().
> > 
> > This depends on whether we want 'assign' to clean up any old notifiers
> > before setting up new ones. I think we want different behaviour for
> > dataplane and vhost.
> 
> I think dataplane and vhost are the same.
> 
> The question is whether ioeventfd=off,vhost=on or
> ioeventfd=off,dataplane=on are valid combinations; I think they aren't.

We should disallow that even temporary, then.

(This would imply that we need to drop the _stop_ioeventfd() call in
->set_host_notifier(), correct?)

> If they aren't, it should be okay to remove the
> virtio_queue_host_notifier_read call in
> virtio_queue_set_host_notifier_fd_handler and
> virtio_queue_aio_set_host_notifier_handler.  That's because a handler
> for the notifier will always be set _somewhere_.  It could be the usual
> ioeventfd handler, the vhost handler or the dataplane handler, but one
> will be there.

It should; but we probably need to do a final read when we stop the
ioeventfd.