Re: [patch uq/master 1/2] virtio-pci: wake up iothread on VIRTIO_PCI_QUEUE_NOTIFY

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Feb 22, 2010 at 04:51:46PM +0200, Avi Kivity wrote:
> On 02/22/2010 04:29 PM, Marcelo Tosatti wrote:
> >On Mon, Feb 22, 2010 at 04:20:52PM +0200, Avi Kivity wrote:
> >>On 02/22/2010 03:59 PM, Marcelo Tosatti wrote:
> >>>VIRTIO_PCI_QUEUE_NOTIFY is used to inform availability of new buffers,
> >>>so wakeup the iothread to process that information immediately.
> >>>
> >>>Reported-by: Amit Shah<amit.shah@redhat.com>
> >>>Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
> >>>
> >>>Index: qemu/hw/virtio-pci.c
> >>>===================================================================
> >>>--- qemu.orig/hw/virtio-pci.c
> >>>+++ qemu/hw/virtio-pci.c
> >>>@@ -204,6 +204,7 @@ static void virtio_ioport_write(void *op
> >>>          break;
> >>>      case VIRTIO_PCI_QUEUE_NOTIFY:
> >>>          virtio_queue_notify(vdev, val);
> >>>+        qemu_notify_event();
> >>>          break;
> >>virtio_queue_notify() will call ->handle_output(), which should
> >>either do what's needed to be done, or wake up some iothread itself.
> >kick is used to inform either output processing, in which case
> >->handle_output() does what its supposed to.
> >
> >But its also used to inform availability of new buffers, which is common
> >to all virtio devices. So what is the point pushing this to
> >->handle_output?
> 
> I don't understand what this means.  ->handle_output() _is_
> informing the device model of new buffers.  What more is needed?
> 
> >Are you concerned about spurious wakeups?
> 
> Yes.  Also, qemu_notify_event() is an undirected notification (wakes
> up all iothreads, and all devices), whereas ->handle_output() is
> directed (wakes up exactly what is needed).
>
> What's the underlying problem?  A new input buffer has become
> available, and we need to re-poll the incoming file descriptor?  If
> so, that's best done from ->handle_output() (either by waking the
> iothread or calling read() itself and perhaps receiving -EAGAIN).

Yes. Sure, perhaps calling read() itself is appropriate, and i see 
your point that >handle_output contains more context for a smarter
decision.

But one can argue thats an improvement on top of a dumb wakeup.