On Wed, Jul 26, 2017 at 09:37:15PM +0800, Jason Wang wrote: > > > On 2017年07月26日 21:18, Jason Wang wrote: > > > > > > On 2017年07月26日 20:57, Michael S. Tsirkin wrote: > > > On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote: > > > > This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it > > > > was reported to break vhost_net. We want to cache used event and use > > > > it to check for notification. We try to valid cached used event by > > > > checking whether or not it was ahead of new, but this is not correct > > > > all the time, it could be stale and there's no way to know about this. > > > > > > > > Signed-off-by: Jason Wang<jasow...@redhat.com> > > > Could you supply a bit more data here please? How does it get stale? > > > What does guest need to do to make it stale? This will be helpful if > > > anyone wants to bring it back, or if we want to extend the protocol. > > > > > > > The problem we don't know whether or not guest has published a new used > > event. The check vring_need_event(vq->last_used_event, new + vq->num, > > new) is not sufficient to check for this. > > > > Thanks > > More notes, the previous assumption is that we don't move used event back, > but this could happen in fact if idx is wrapper around.
You mean if the 16 bit index wraps around after 64K entries. Makes sense. > Will repost and add > this into commit log. > > Thanks
