Fix a race condition where qemu finds that there are not enough virtio
ring buffers available and the guest make more buffers available before
qemu can enable notifications.
Signed-off-by: Tom Lendacky <toml@xxxxxxxxxx>
Signed-off-by: Anthony Liguori <aliguori@xxxxxxxxxx>
hw/virtio-net.c | 10 +++++++++-
1 files changed, 9 insertions(+), 1 deletions(-)
diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 6e48997..5c0093e 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -379,7 +379,15 @@ static int virtio_net_has_buffers(VirtIONet *n, int bufsize)
(n->mergeable_rx_bufs &&
!virtqueue_avail_bytes(n->rx_vq, bufsize, 0))) {
virtio_queue_set_notification(n->rx_vq, 1);
- return 0;
+
+ /* To avoid a race condition where the guest has made some buffers
+ * available after the above check but before notification was
+ * enabled, check for available buffers again.
+ */
+ if (virtio_queue_empty(n->rx_vq) ||
+ (n->mergeable_rx_bufs &&
+ !virtqueue_avail_bytes(n->rx_vq, bufsize, 0)))
+ return 0;
}
virtio_queue_set_notification(n->rx_vq, 0);
On Friday 29 January 2010 02:06:41 pm Tom Lendacky wrote:
> There's been some discussion of this already in the kvm list, but I want to
> summarize what I've found and also include the qemu-devel list in an effort
> to find a solution to this problem.
>
> Running a netperf test between two kvm guests results in the guest's
> network interface shutting down. I originally found this using kvm guests
> on two different machines that were connected via a 10GbE link. However,
> I found this problem can be easily reproduced using two guests on the same
> machine.
>
> I am running the 2.6.32 level of the kvm.git tree and the 0.12.1.2 level of
> the qemu-kvm.git tree.
>
> The setup includes two bridges, br0 and br1.
>
> The commands used to start the guests are as follows:
> usr/local/bin/qemu-system-x86_64 -name cape-vm001 -m 1024 -drive
> file=/autobench/var/tmp/cape-vm001-
> raw.img,if=virtio,index=0,media=disk,boot=on -net
> nic,model=virtio,vlan=0,macaddr=00:16:3E:00:62:51,netdev=cape-vm001-eth0 -
> netdev tap,id=cape-vm001-eth0,script=/autobench/var/tmp/ifup-kvm-
> br0,downscript=/autobench/var/tmp/ifdown-kvm-br0 -net
> nic,model=virtio,vlan=1,macaddr=00:16:3E:00:62:D1,netdev=cape-vm001-eth1 -
> netdev tap,id=cape-vm001-eth1,script=/autobench/var/tmp/ifup-kvm-
> br1,downscript=/autobench/var/tmp/ifdown-kvm-br1 -vnc :1 -monitor
> telnet::5701,server,nowait -snapshot -daemonize
>
> usr/local/bin/qemu-system-x86_64 -name cape-vm002 -m 1024 -drive
> file=/autobench/var/tmp/cape-vm002-
> raw.img,if=virtio,index=0,media=disk,boot=on -net
> nic,model=virtio,vlan=0,macaddr=00:16:3E:00:62:61,netdev=cape-vm002-eth0 -
> netdev tap,id=cape-vm002-eth0,script=/autobench/var/tmp/ifup-kvm-
> br0,downscript=/autobench/var/tmp/ifdown-kvm-br0 -net
> nic,model=virtio,vlan=1,macaddr=00:16:3E:00:62:E1,netdev=cape-vm002-eth1 -
> netdev tap,id=cape-vm002-eth1,script=/autobench/var/tmp/ifup-kvm-
> br1,downscript=/autobench/var/tmp/ifdown-kvm-br1 -vnc :2 -monitor
> telnet::5702,server,nowait -snapshot -daemonize
>
> The ifup-kvm-br0 script takes the (first) qemu created tap device and
> brings it up and adds it to bridge br0. The ifup-kvm-br1 script take the
> (second) qemu created tap device and brings it up and adds it to bridge
> br1.
>
> Each ethernet device within a guest is on it's own subnet. For example:
> guest 1 eth0 has addr 192.168.100.32 and eth1 has addr 192.168.101.32
> guest 2 eth0 has addr 192.168.100.64 and eth1 has addr 192.168.101.64
>
> On one of the guests run netserver:
> netserver -L 192.168.101.32 -p 12000
>
> On the other guest run netperf:
> netperf -L 192.168.101.64 -H 192.168.101.32 -p 12000 -t TCP_STREAM -l 60
> -c -C -- -m 16K -M 16K
>
> It may take more than one netperf run (I find that my second run almost
> always causes the shutdown) but the network on the eth1 links will stop
> working.
>
> I did some debugging and found that in qemu on the guest running netserver:
> - the receive_disabled variable is set and never gets reset
> - the read_poll event handler for the eth1 tap device is disabled and
> never re-enabled
> These conditions result in no packets being read from the tap device and
> sent to the guest - effectively shutting down the network. Network
> connectivity can be restored by shutting down the guest interfaces,
> unloading the virtio_net module, re-loading the virtio_net module and
> re-starting the guest interfaces.
>
> I'm continuing to work on debugging this, but would appreciate if some
> folks with more qemu network experience could try to recreate and debug
> this.
>
> If my kernel config matters, I can provide that.
>
> Thanks,
> Tom
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