[Qemu-devel] networking stalls in the guest -- backlog in the host

[David Ahern <dsahern@gmail.com>]

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