From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46089) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZAhdE-0003Vi-QX for qemu-devel@nongnu.org; Thu, 02 Jul 2015 12:46:40 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZAhdA-0001Vd-NY for qemu-devel@nongnu.org; Thu, 02 Jul 2015 12:46:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53128) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZAhdA-0001V4-IG for qemu-devel@nongnu.org; Thu, 02 Jul 2015 12:46:32 -0400 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) by mx1.redhat.com (Postfix) with ESMTPS id E650B344F54 for ; Thu, 2 Jul 2015 16:46:31 +0000 (UTC) Date: Thu, 2 Jul 2015 18:46:28 +0200 From: "Michael S. Tsirkin" Message-ID: <20150702184359-mutt-send-email-mst@redhat.com> References: <1435633611-14023-1-git-send-email-famz@redhat.com> <559254CC.4000106@redhat.com> <20150702124626.GE21214@stefanha-thinkpad.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150702124626.GE21214@stefanha-thinkpad.home> Subject: Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Jason Wang , Fam Zheng , qemu-devel@nongnu.org On Thu, Jul 02, 2015 at 01:46:26PM +0100, Stefan Hajnoczi wrote: > On Tue, Jun 30, 2015 at 04:35:24PM +0800, Jason Wang wrote: > > On 06/30/2015 11:06 AM, Fam Zheng wrote: > > > virtio_net_receive still does the check by calling > > > virtio_net_can_receive, if the device or driver is not ready, the packet > > > is dropped. > > > > > > This is necessary because returning false from can_receive complicates > > > things: the peer would disable sending until we explicitly flush the > > > queue. > > > > > > Signed-off-by: Fam Zheng > > > --- > > > hw/net/virtio-net.c | 1 - > > > 1 file changed, 1 deletion(-) > > > > > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c > > > index d728233..dbef0d0 100644 > > > --- a/hw/net/virtio-net.c > > > +++ b/hw/net/virtio-net.c > > > @@ -1503,7 +1503,6 @@ static int virtio_net_load_device(VirtIODevice *vdev, QEMUFile *f, > > > static NetClientInfo net_virtio_info = { > > > .type = NET_CLIENT_OPTIONS_KIND_NIC, > > > .size = sizeof(NICState), > > > - .can_receive = virtio_net_can_receive, > > > .receive = virtio_net_receive, > > > .link_status_changed = virtio_net_set_link_status, > > > .query_rx_filter = virtio_net_query_rxfilter, > > > > A side effect of this patch is it will read and then drop packet is > > guest driver is no ok. > > I think that the semantics of .can_receive() and .receive() return > values are currently incorrect in many NICs. They have .can_receive() > functions that return false for conditions where .receive() would > discard the packet. So what happens is that packets get queued when > they should actually be discarded. > > The purpose of the flow control (queuing) mechanism is to tell the > sender to hold off until the receiver has more rx buffers available. > It's a short-term thing that doesn't included link down, rx disable, or > NIC reset states. > > Therefore, I think this patch will not introduce a regression. It is > adjusting the code to stop queuing packets when they should actually be > dropped. > > Thoughts? > > Reviewed-by: Stefan Hajnoczi OK, but we do have transient out of buffers states too, and virtio_net_can_receive checks these as well. To me, it looks like we should change virtio_net_can_receive to avoid checking link down state, etc. But I don't see why we should remove it completely. -- MST