From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by dpdk.org (Postfix) with ESMTP id 6C9982BCC for ; Wed, 25 May 2016 11:50:07 +0200 (CEST) Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D33B76406C; Wed, 25 May 2016 09:50:06 +0000 (UTC) Received: from redhat.com (vpn1-7-192.ams2.redhat.com [10.36.7.192]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with SMTP id u4P9o31Y021176; Wed, 25 May 2016 05:50:03 -0400 Date: Wed, 25 May 2016 12:50:02 +0300 From: "Michael S. Tsirkin" To: Bruce Richardson Cc: "Xie, Huawei" , "dev@dpdk.org" , "stephen@networkplumber.org" , "Ananyev, Konstantin" , "thomas.monjalon@6wind.com" , Yuanhan Liu , "Tan, Jianfeng" Message-ID: <20160525124815-mutt-send-email-mst@redhat.com> References: <1464106601-981-1-git-send-email-huawei.xie@intel.com> <20160525113224-mutt-send-email-mst@redhat.com> <20160525094729.GA4256@bricha3-MOBL3> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160525094729.GA4256@bricha3-MOBL3> X-Scanned-By: MIMEDefang 2.68 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Wed, 25 May 2016 09:50:06 +0000 (UTC) Subject: Re: [dpdk-dev] [PATCH] virtio: use volatile to get used->idx in the loop X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 25 May 2016 09:50:07 -0000 On Wed, May 25, 2016 at 10:47:30AM +0100, Bruce Richardson wrote: > On Wed, May 25, 2016 at 11:34:24AM +0300, Michael S. Tsirkin wrote: > > On Wed, May 25, 2016 at 08:25:20AM +0000, Xie, Huawei wrote: > > > On 5/25/2016 4:12 PM, Xie, Huawei wrote: > > > > There is no external function call or any barrier in the loop, > > > > the used->idx would only be retrieved once. > > > > > > > > Signed-off-by: Huawei Xie > > > > --- > > > > drivers/net/virtio/virtio_ethdev.c | 3 ++- > > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c > > > > index c3fb628..f6d6305 100644 > > > > --- a/drivers/net/virtio/virtio_ethdev.c > > > > +++ b/drivers/net/virtio/virtio_ethdev.c > > > > @@ -204,7 +204,8 @@ virtio_send_command(struct virtqueue *vq, struct virtio_pmd_ctrl *ctrl, > > > > usleep(100); > > > > } > > > > > > > > - while (vq->vq_used_cons_idx != vq->vq_ring.used->idx) { > > > > + while (vq->vq_used_cons_idx != > > > > + *((volatile uint16_t *)(&vq->vq_ring.used->idx))) { > > > > uint32_t idx, desc_idx, used_idx; > > > > struct vring_used_elem *uep; > > > > > > > > > > Find this issue when do the code rework of RX/TX queue. > > > As in other places, we also have loop retrieving the value of avial->idx > > > or used->idx, i prefer to declare the index in vq structure as volatile > > > to avoid potential issue. > > Is there a reason why the value is not always volatile? I would have thought > it would be generally safer to mark the actual value as volatile inside the > structure definition itself? In any cases where we do want to store the value > locally and not re-access the structure, a local variable can be used. > > Regards, > /Bruce Linux generally discourages volatile as a general style guidance: https://www.kernel.org/doc/Documentation/volatile-considered-harmful.txt it doesn't have to apply to dpdk which has a different coding style but IIUC this structure is inherited from linux, deviating will make keeping things up to date harder. > > > > It might be a good idea to wrap this in a macro > > similar to ACCESS_ONCE in Linux. > > > > > > > > Stephen: > > > Another question is why we need a loop here? > > > > > > /huawei > > > > -- > > MST