From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH 2/3] vhost: better detection of available buffers Date: Tue, 15 Nov 2016 16:00:21 +0800 Message-ID: <8bf86752-4dca-3ced-4641-efa7a4a1fc6e@redhat.com> 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> <6f650bf0-1e2b-1c35-db34-5024383a6892@redhat.com> <20161115052753-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: "Michael S. Tsirkin" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:33916 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934210AbcKOIAZ (ORCPT ); Tue, 15 Nov 2016 03:00:25 -0500 In-Reply-To: <20161115052753-mutt-send-email-mst@kernel.org> Sender: netdev-owner@vger.kernel.org List-ID: On 2016年11月15日 11:28, Michael S. Tsirkin wrote: > On Tue, Nov 15, 2016 at 11:16:59AM +0800, Jason Wang wrote: >> >> 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. > Absolutely but it checks once and then only re-checks after value > changes again. > Since get_rx_bufs() does not get enough buffers, we will wait for the kick in this case. For busy polling, we probably want to stay in the busy loop here.