On 16/03/2016 14:04, Cornelia Huck wrote:
> > No, it would not. ioeventfd=off,vhost=on would mean: "when vhost is
> > off, use vCPU thread notification".
>
> *confused*
>
> Is ioeventfd=off supposed to mean "don't talk to the kernel, do
> everything in qemu"?
For KVM, it means do everything in the QEMU vCPU thread (using userspace
vmexits).
>> > When turning on vhost you'd still stop ioeventfd (i.e. stop processing
>> > the virtqueue in QEMU's main iothread), but you don't need to do
>> > anything to the event notifier. vhost will pick it up and work on the
>> > virtqueue if necessary. Likewise for dataplane.
>
> So "disassociate the handler and switch over to the new one"?
Yes, if we prohibit combinations which switch from vCPU thread
notification to vhost or dataplane (such as ioeventfd=off,vhost=on). If
we always use an eventfd, we always have a handler to switch to.
> > > > 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.
> >
> > I was thinking of handing the final read directly to the next guy who
> > polls the event notifier instead. So, when called from vhost or
> > dataplane, virtio_pci_stop_ioeventfd would use
> > assign=true/set_handler=false ("a new notifier is going to be set up by
> > the caller").
>
> OK, then we'd need to pass a new parameter for this.
Yes, agreed.
> > The host notifier API unfortunately is full of indirections. I'm not
> > sure how many of them are actually necessary.
>
> Oh yes, it's very hard to follow, especially with not-very-well defined
> parameters.
And full of duplicate code. If copied code were moved to the virtio bus
level, it would be easier to change too.
Paolo
