Re: Wrong TCP checksum of packets sent by Linux guest (virtIO/vhost)

[Franck Baudin <franck.baudin-kQLz4VG1Jc7QT0dZR+AlfA@public.gmane.org>]

Hi,

On 09/03/14 13:13, Xie, Huawei wrote:
> Looping in the dpdk-ovs list.
>
> * Does the new vhost API allow a user to know if all the relevant offloads have
> been
> turned on/off for that interface? It seems that this is possible through the
> virtio_net
> structure but it would be good to get some feedback from the relevant person
> working on DPDK (Huawei?).
>
> * If this is the case, then it is probably in the realm of the vswitch do the actual
> checksum (for VM-VM) or correctly configure the NIC when sending out through
> the physical interface.
>
> Comments?
>
> Mark:
> So far not supported. This is important as well in VxLan case. For the packet flow
> Guest A->  virtio -> ..->OVDK->.. -> Guest B.
> 1) If guest A and B are on different host machines, say A and B respectively,  and if the nic on A supports
> vxlan checksum offload, then both guest and host needn't generate checksum, the nic will
> generate checksum for both inner and outer packet.
> 2) In VM2VM case, as it is trusted communication channel, could we negotiate with the guest tcp stack not to verify checksum
> for received packet?
The problem is that any TCP packet send by a vanilla Linux guest through 
vhost is incorrect (VM to anything, including other colocalied VMs). In 
other words, the VM cannot use TCP. QEMU options and ethtool -K csum off 
tso off ("TCP stack negociation") have no effect, maybe because the 
vhost backend is misbehaving.

Franck