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Date: Mon, 12 Aug 2013 17:10:11 +0300
From: "Michael S. Tsirkin" <mst@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: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

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.