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Date: Tue, 13 Aug 2013 10:14:11 +0200
From: Stefan Hajnoczi <stefanha@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH for-1.6] virtio: clear signalled_used_valid
 when switching from dataplane
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>

On Mon, Aug 12, 2013 at 05:10:11PM +0300, Michael S. Tsirkin wrote:
> On Mon, Aug 12, 2013 at 03:39:23PM +0200, Stefan Hajnoczi wrote:
> > On Mon, Aug 12, 2013 at 11:18 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > On Mon, Aug 12, 2013 at 11:08:09AM +0200, Stefan Hajnoczi wrote:
> > >> When the dataplane thread stops, its vring.c implementation synchronizes
> > >> vring state back to virtio.c so we can continue emulating the virtio
> > >> device.
> > >>
> > >> This patch ensures that virtio.c's signalled_used_valid flag is reset so
> > >> that we do not suppress guest notifications due to stale signalled_used
> > >> values.
> > >>
> > >> Suggested-by: Kevin Wolf <kwolf@redhat.com>
> > >> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > >
> > > Good point
> > >
> > > Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > >
> > > and we also need this for vhost.c right?
> > 
> > I think vhost is not affected by this because virtio.c clears the
> > signalled_used_valid flag when the device is reset.  Since vhost_net
> > does not transition back and forth between userspace virtio and vhost
> > (without a device reset), there is no need to reset the flag.
> > 
> > Stefan
> 
> Yes it does transition on vm stop/start.

I see.  Come to think of it maybe dataplane could use virtio.c's vm
change state handler instead of registering its own.  Anyway, thanks for
pointing this out.

Stefan