Re: [PATCH net] Revert "vhost: cache used event for better performance"

[Jason Wang]

On 2017å07æ26æ 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<jasowang@xxxxxxxxxx>
> 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