From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60271) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WTIYD-0006Xv-6n for qemu-devel@nongnu.org; Thu, 27 Mar 2014 18:13:37 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WTIY4-0007yC-9a for qemu-devel@nongnu.org; Thu, 27 Mar 2014 18:13:29 -0400 Received: from mail-pb0-x235.google.com ([2607:f8b0:400e:c01::235]:61110) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WTIY4-0007y3-2T for qemu-devel@nongnu.org; Thu, 27 Mar 2014 18:13:20 -0400 Received: by mail-pb0-f53.google.com with SMTP id rp16so4073248pbb.40 for ; Thu, 27 Mar 2014 15:13:19 -0700 (PDT) Message-ID: <5334A27B.2000002@gmail.com> Date: Thu, 27 Mar 2014 16:13:15 -0600 From: David Ahern MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] networking stalls in the guest -- backlog in the host List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org We are hitting a networking problem and hoping someone has an idea -- perhaps a known bug. After a couple of hours of runtime with low level traffic (e.g., 1 sec pings) the VM stops receiving packets. In the host running tc on the tap device shows a full backlog and packets getting dropped: tc -s qdisc show dev vnet0 qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1 Sent 5806496634 bytes 4163358 pkt (dropped 116079, overlimits 0 requeues 4) backlog 33834b 500p requeues 4 The tap device is passed to qemu as fd=24. Running strace on the IO thread does not show the fd in the list passed to select. e.g., select(55, [7 8 11 18 52 53 54], [], [], {1, 0}) = 1 (in [8], left {0, 872402}) That would explain why the packets are not pulled from the tap device into the VM. When networking is functioning properly, you do see fd=24 in the list followed by read(24, ...). Why would qemu stop adding the fd to the list passed to select? This is qemu-kvm-1.0 (upgrading is not an option), started by libvirt (libvirt 1.0.2). The command line is rather long. Snippets: /usr/bin/kvm -M pc-1.0 -cpu host -enable-kvm -m 4096 -smp 4 ... -netdev tap,fd=24,id=hostnet0 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=00:16:3e:ba:55:60,bus=pci.0,addr=0xa ... Host kernel: 3.2.0-60-generic Guest kernel: 3.8.0-29-generic Thanks, David