[ovs-dev] [PATCH v4 0/3] Add support for TSO with DPDK

[Flavio Leitner]

On Tue, Jan 28, 2020 at 03:23:02PM -0800, Yifeng Sun wrote:
> Sure.
> 
> Firstly, make sure userspace-tso-enable is true
> # ovs-vsctl get Open_vSwitch . other_config
> {dpdk-init="true", enable-statistics="true", pmd-cpu-mask="0xf",
> userspace-tso-enable="true"}
> 
> Next, create 2 VMs with vhostuser-type interface on the same KVM host:
>     <interface type='vhostuser'>
>       <mac address='88:69:00:00:00:11'/>
>       <source type='unix'
> path='/tmp/041afca0-6e11-4eab-a62f-1ccf5cd318fd' mode='server'/>
>       <model type='virtio'/>
>       <driver queues='2' rx_queue_size='512'>
>         <host csum='on' tso4='on' tso6='on'/>
>         <guest csum='on' tso4='on' tso6='on'/>

I have other options set, but I don't think they are related:
       <host csum='on' gso='off' tso4='on' tso6='on' ecn='off'
ufo='off' mrg_rxbuf='on'/>
       <guest csum='on' tso4='on' tso6='on' ecn='off' ufo='off'/>


>       </driver>
>       <alias name='net2'/>
>       <address type='pci' domain='0x0000' bus='0x00' slot='0x06'
> function='0x0'/>
>     </interface>
> 
> When VM boots up, turn on tx, tso and sg
> # ethtool -K ens6 tx on
> # ethtool -K ens6 tso on
> # ethtool -K ens6 sg on

All the needed offloading features are turned on by default,
so I don't change anything in my testbed.

> Then run 'iperf -s' on one VM and 'iperf -c xx.xx.xx.xx' on another VM.
> Iperf doesn't work if there is no chage to VM's kernel. `tcpdump` shows
> that iperf server received packets with invalid TCP checksum.
> `nstat -a` shows that TcpInCsumErr number is accumulating.
> 
> After adding changes to VM's kernel as below, iperf works properly.
> in tcp_v4_rcv()
>       - if (skb_checksum_init(skb, IPPROTO_TCP, inet_compute_pseudo))
>       + if (skb_checksum_init(skb, IPPROTO_TCP, inet_compute_pseudo))
> 
> static inline bool tcp_checksum_complete(struct sk_buff *skb)
> {
>         return 0;
> }

That's odd. Which kernel is that? Maybe I can try the same version.
I am using 5.2.14-200.fc30.x86_64.

Looks like somehow the packet lost its offloading flags, then kernel
has to check the csum and since it wasn't calculated before, it's 
just random garbage.

fbl


> 
> 
> 
> Best,
> Yifeng
> 
> On Tue, Jan 28, 2020 at 2:52 PM Flavio Leitner <fbl at sysclose.org> wrote:
> >
> > On Tue, Jan 28, 2020 at 02:21:30PM -0800, Yifeng Sun wrote:
> > > Hi Flavio,
> > >
> > > Thanks for the explanation. I followed the steps in the document but
> > > TCP connection still failed to build between 2 VMs.
> > >
> > > I finally modified VM's kernel directly to disable TCP checksum validation
> > > to get it working properly. I got 30.0Gbps for 'iperf' between 2 VMs.
> >
> > Could you provide more details on how you did that? What's running
> > inside the VM?
> >
> > I don't change anything inside of the VMs (Linux) in my testbed.
> >
> > fbl
> >
> >
> > >
> > > Best,
> > > Yifeng
> > >
> > >
> > > On Tue, Jan 28, 2020 at 4:00 AM Flavio Leitner <fbl at sysclose.org> wrote:
> > > >
> > > > On Mon, Jan 27, 2020 at 05:17:01PM -0800, Yifeng Sun wrote:
> > > > > Hi Ilya,
> > > > >
> > > > > Thanks for your reply.
> > > > >
> > > > > The thing is, if checksum offloading is enabled in both VMs, then
> > > > > sender VM will send
> > > > > a packet with invalid TCP checksum, and later OVS will send this
> > > > > packet to receiver
> > > > > VM directly without calculating a valid checksum. As a result,
> > > > > receiver VM will drop
> > > > > this packet because it contains invalid checksum. This is what
> > > > > happened when I tried
> > > > > this patch.
> > > > >
> > > >
> > > > When TSO is enabled, the TX checksumming offloading is required,
> > > > then you will see invalid checksum. This is well documented here:
> > > >
> > > > https://github.com/openvswitch/ovs/blob/master/Documentation/topics/userspace-tso.rst#userspace-datapath---tso
> > > >
> > > > "Additionally, if the traffic is headed to a VM within the same host
> > > > further optimization can be expected. As the traffic never leaves
> > > > the machine, no MTU needs to be accounted for, and thus no
> > > > segmentation and checksum calculations are required, which saves yet
> > > > more cycles."
> > > >
> > > > Therefore, it's expected to see bad csum in the traffic dumps.
> > > >
> > > > To use the feature, you need few steps: enable the feature in OvS
> > > > enable in qemu and inside the VM. The linux guest usually enable
> > > > the feature by default if qemu offers it.
> > > >
> > > > HTH,
> > > > fbl
> > > >
> > > >
> > > > > Best,
> > > > > Yifeng
> > > > >
> > > > > On Mon, Jan 27, 2020 at 12:09 PM Ilya Maximets <i.maximets at ovn.org> wrote:
> > > > > >
> > > > > > On 27.01.2020 18:24, Yifeng Sun wrote:
> > > > > > > Hi Flavio,
> > > > > > >
> > > > > > > I am testing your patch using iperf between 2 VMs on the same host.
> > > > > > > But it seems that TCP connection can't be created between these 2 VMs.
> > > > > > > When inspecting further, I found that TCP packets have invalid checksums.
> > > > > > > This might be the reason.
> > > > > > >
> > > > > > > I am wondering if I missed something in the setup? Thanks a lot.
> > > > > >
> > > > > > I didn't test myself, but according to current design, checksum offloading
> > > > > > (rx and tx) shuld be enabled in both VMs.  Otherwise all the packets will
> > > > > > be dropped by the guest kernel.
> > > > > >
> > > > > > Best regards, Ilya Maximets.
> > > >
> > > > --
> > > > fbl
> >
> > --
> > fbl

-- 
fbl