From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965393AbcKODRH (ORCPT ); Mon, 14 Nov 2016 22:17:07 -0500 Received: from mx1.redhat.com ([209.132.183.28]:40542 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965000AbcKODRF (ORCPT ); Mon, 14 Nov 2016 22:17:05 -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> <20161111182023-mutt-send-email-mst@kernel.org> Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org From: Jason Wang Message-ID: <6f650bf0-1e2b-1c35-db34-5024383a6892@redhat.com> Date: Tue, 15 Nov 2016 11:16:59 +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: <20161111182023-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 [10.5.110.26]); Tue, 15 Nov 2016 03:17:04 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2016年11月12日 00:20, Michael S. Tsirkin wrote: > On Fri, Nov 11, 2016 at 12:18:50PM +0800, Jason Wang wrote: >> >> 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. > What's wrong with what we have? It polls until value changes. > But as you said, this does not mean (in mergeable cases) we have enough space to store the packet.