From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751867AbdGZNhe (ORCPT ); Wed, 26 Jul 2017 09:37:34 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35558 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751653AbdGZNhc (ORCPT ); Wed, 26 Jul 2017 09:37:32 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com E00EFC0587D1 Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=jasowang@redhat.com Subject: Re: [PATCH net] Revert "vhost: cache used event for better performance" From: Jason Wang To: "Michael S. Tsirkin" Cc: virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <1501056197-3368-1-git-send-email-jasowang@redhat.com> <20170726155435-mutt-send-email-mst@kernel.org> <117542fc-eef9-c043-7c9e-daafceb7db4e@redhat.com> Message-ID: <6b3a9a98-c095-1729-3528-dd521f136797@redhat.com> Date: Wed, 26 Jul 2017 21:37:15 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <117542fc-eef9-c043-7c9e-daafceb7db4e@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Wed, 26 Jul 2017 13:37:32 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >> 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. Will repost and add this into commit log. Thanks