On Wed, 2017-07-26 at 19:08 +0300, Michael S. Tsirkin wrote: > 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<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 > > > > 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 Hi, I am just curious but I have got a question: AFAIU, if you wanted to keep the caching mechanism alive in the code base, the following two changes could clear off the issue, or not?: (1) Always fetch the latest event value from guest when signalled_used event is invalid, which includes last_used_idx wraps-around case. Otherwise we might need changes which would complicate too much the logic to properly decide whether or not to skip signalling in the next vhost_notify round. (2) On top of that, split the signal-postponing logic to three cases like: * if the interval of vq.num is [2^16, UINT_MAX]: any cached event is in should-postpone-signalling interval, so paradoxically must always do signalling. * else if the interval of vq.num is [2^15, 2^16): the logic in the original patch (809ecb9bca6a9) suffices * else (= less than 2^15) (optional): checking only (vring_need_event(vq->last_used_event, new + vq->num, new) would suffice. Am I missing something, or is this irrelevant? I would appreciate if you could elaborate a bit more how the situation where event idx wraps around and moves back would make trouble. Thanks. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
