From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966509AbcKKES4 (ORCPT ); Thu, 10 Nov 2016 23:18:56 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51570 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965482AbcKKESz (ORCPT ); Thu, 10 Nov 2016 23:18:55 -0500 Subject: Re: [PATCH 2/3] vhost: better detection of available buffers To: "Michael S. Tsirkin" References: <1478677113-13126-1-git-send-email-jasowang@redhat.com> <1478677113-13126-2-git-send-email-jasowang@redhat.com> <20161109215513-mutt-send-email-mst@kernel.org> <70cb5ad8-5266-b0c4-7b55-4aea4f5f01a6@redhat.com> <20161111054021-mutt-send-email-mst@kernel.org> Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org From: Jason Wang Message-ID: Date: Fri, 11 Nov 2016 12:18:50 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <20161111054021-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Fri, 11 Nov 2016 04:18:54 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2016年11月11日 11:41, Michael S. Tsirkin wrote: > On Fri, Nov 11, 2016 at 10:18:37AM +0800, Jason Wang wrote: >> > >> > >> >On 2016年11月10日 03:57, Michael S. Tsirkin wrote: >>> > >On Wed, Nov 09, 2016 at 03:38:32PM +0800, Jason Wang wrote: >>>> > > >We should use vq->last_avail_idx instead of vq->avail_idx in the >>>> > > >checking of vhost_vq_avail_empty() since latter is the cached avail >>>> > > >index from guest but we want to know if there's pending available >>>> > > >buffers in the virtqueue. >>>> > > > >>>> > > >Signed-off-by: Jason Wang >>> > >I'm not sure why is this patch here. Is it related to >>> > >batching somehow? >> > >> >Yes, we need to know whether or not there's still buffers left in the >> >virtqueue, so need to check last_avail_idx. Otherwise, we're checking if >> >guest has submitted new buffers. >> > >>> > > >>> > > >>>> > > >--- >>>> > > > drivers/vhost/vhost.c | 2 +- >>>> > > > 1 file changed, 1 insertion(+), 1 deletion(-) >>>> > > > >>>> > > >diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >>>> > > >index c6f2d89..fdf4cdf 100644 >>>> > > >--- a/drivers/vhost/vhost.c >>>> > > >+++ b/drivers/vhost/vhost.c >>>> > > >@@ -2230,7 +2230,7 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq) >>>> > > > if (r) >>>> > > > return false; >>>> > > >- return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx; >>>> > > >+ return vhost16_to_cpu(vq, avail_idx) == vq->last_avail_idx; >>>> > > > } >>>> > > > EXPORT_SYMBOL_GPL(vhost_vq_avail_empty); >>> > >That might be OK for TX but it's probably wrong for RX >>> > >where the fact that used != avail does not mean >>> > >we have enough space to store the packet. >> > >> >Right, but it's no harm since it was just a hint, handle_rx() can handle >> >this situation. > Means busy polling will cause useless load on the CPU though. > Right, but,it's not easy to have 100% correct hint here. Needs more thought.