From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH net] Revert "vhost: cache used event for better performance" Date: Wed, 26 Jul 2017 21:18:02 +0800 Message-ID: <117542fc-eef9-c043-7c9e-daafceb7db4e@redhat.com> References: <1501056197-3368-1-git-send-email-jasowang@redhat.com> <20170726155435-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20170726155435-mutt-send-email-mst@kernel.org> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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 > 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