Patchwork Re: Network shutdown under load

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Submitter Tom Lendacky
Date Feb. 8, 2010, 4:10 p.m.
Message ID <201002081010.03751.tahm@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/44802/
State New
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Tom Lendacky - Feb. 8, 2010, 4:10 p.m.
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@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

 hw/virtio-net.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)


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
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
Anthony Liguori - Feb. 8, 2010, 8:58 p.m.
On 02/08/2010 10:10 AM, Tom Lendacky wrote:
> 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@us.ibm.com>
> Signed-off-by: Anthony Liguori<aliguori@us.ibm.com>
>    

I've walked through the changes in this series and I'm pretty certain 
that this is the only problem.  I'd appreciate if others could review 
though.

Regards,

Anthony Liguori

>   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
>> --
>> To unsubscribe from this list: send the line "unsubscribe kvm" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>>
Herbert Xu - Feb. 8, 2010, 9:18 p.m.
On Mon, Feb 08, 2010 at 10:10:01AM -0600, Tom Lendacky wrote:
> 
> 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@us.ibm.com>
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

Good cath!
 
> 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.
> +         */

We should also add a full memory barrier right here to avoid
out-of-order loads.

> +        if (virtio_queue_empty(n->rx_vq) ||
> +            (n->mergeable_rx_bufs &&
> +             !virtqueue_avail_bytes(n->rx_vq, bufsize, 0)))
> +            return 0;
>      }

Cheers,
RW - Feb. 9, 2010, 8:29 p.m.
Thanks for the patch! It seems to solve the problem that
under load (> 50 MBit/s) the network goes down. I've applied
the patch to KVM 0.12.2 running Gentoo. Host and guest is running
kernel 2.6.32 currently (kernel 2.6.30 in guest and 2.6.32 in
host works also for us).

Another host doing the same jobs with the same amount of traffic
and configuration but with KVM 0.11.1 was shutting down
the network interface every 5-10 minutes today while the
patched 0.12.2 was running fine. During the time the 0.11.1 KVMs
were down the patched one delivered >200 MBit/s without
problems. Now both hosts running with the patched
version. We're expecting much more traffic tomorrow so if the
network is still up on thursday I would say the bug is fixed.

Thanks for that patch! It really was a lifesaver today :-)

- Robert


On 02/08/2010 05:10 PM, Tom Lendacky wrote:
> 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@us.ibm.com>
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>
>  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
>> --
>> To unsubscribe from this list: send the line "unsubscribe kvm" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>>     
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
Anthony Liguori - Feb. 10, 2010, 7:31 p.m.
On 02/08/2010 10:10 AM, Tom Lendacky wrote:
> 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@us.ibm.com>
> Signed-off-by: Anthony Liguori<aliguori@us.ibm.com>
>    

Applied.  Thanks.  We should audit the code for proper barrier support.  
Right now, I think there's a lot of places that we're missing them.

Regards,

Anthony Liguori

>   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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>>
>>      
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>

Patch

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);