From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751533AbdGZNSK (ORCPT ); Wed, 26 Jul 2017 09:18:10 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36910 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750781AbdGZNSJ (ORCPT ); Wed, 26 Jul 2017 09:18:09 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com D4259285D0 Authentication-Results: ext-mx06.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx06.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" 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> From: Jason Wang Message-ID: <117542fc-eef9-c043-7c9e-daafceb7db4e@redhat.com> Date: Wed, 26 Jul 2017 21:18:02 +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: <20170726155435-mutt-send-email-mst@kernel.org> 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 [10.5.110.30]); Wed, 26 Jul 2017 13:18:09 +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 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