Implementing a simple TAP PMD to dpdk-vhost structure

[Maxime Coquelin]

On 9/6/23 10:28, Nicolson Ken (ニコルソン ケン) wrote:
> Hi Maxime,
> 
>> Maybe you could use testpmd application instead, with net_tap PMD and a net_pcap PMD?
> 
> I have an existing solution that uses the TAP PMD which I then add to a standard kernel bridge that allows me to have two-way communication with the Guest VM, but I suspect the performance would be better if we were to use a DPDK virtio-based solution; otherwise I could just drop DPDK all together and use tcpreplay, etc to directly access the bridge.

Ha yes, I misread the initial thread.
You should indeed use testpmd with Vhost PMD and net_pcap, I'm pretty
sure I've done that in the past.

> 
> [Also net_pcap only outputs to kernel interface (or to a file or null) as it uses libpcap APIs for Tx]
> 
>> An alternative to net_tap could be to use Virtio-user PMD with Vhost- kernel backend.
> 
> That uses KNI, which the documentation says is deprecated, and I'm not sure I want to start mucking about with kernel drivers.

Virtio-user PMD with kernel backend is an alternative solution to KNI,
no out-of-tree driver needed. But that should not be the way to go for
your usecase.

Maxime
> Thanks,
> Ken
> 
> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin at redhat.com>
> Sent: Wednesday, September 6, 2023 4:16 PM
> To: David Marchand <david.marchand at redhat.com>; Nicolson Ken (ニコルソン ケン) <ken.nicolson at jp.panasonic.com>
> Cc: users at dpdk.org; Xia, Chenbo <chenbo.xia at intel.com>
> Subject: Re: Implementing a simple TAP PMD to dpdk-vhost structure
> 
> 
> 
> On 9/6/23 08:07, David Marchand wrote:
>> Hello Ken,
>>
>> On Wed, Sep 6, 2023 at 3:56 AM Nicolson Ken (ニコルソン ケン)
>> <ken.nicolson at jp.panasonic.com> wrote:
>>>
>>> Hi all,
>>>
>>> Using dpdk 22.11.2 on Ubuntu 22.04
>>>
>>> I have a really simple use case, but I cannot find how to implement it. I've set up QEMU with all the required virtio support, so I just need to configure my Host OS-side. I want to send data from a PCAP file via tcpreplay from the Host to the Guest, so I use this command line:
>>>
>>> $ sudo /home/integ/dpdk-stable-22.11.2/build/examples/dpdk-vhost -l
>>> 0-3 -n 4 --socket-mem 1024 --vdev 'net_tap0' -- --socket-file
>>> /tmp/sock0 --client -p 1
>>>
>>> However, this fails with:
>>>
>>> EAL: Detected CPU lcores: 20
>>> EAL: Detected NUMA nodes: 1
>>> EAL: Detected static linkage of DPDK
>>> EAL: Multi-process socket /var/run/dpdk/rte/mp_socket
>>> EAL: Selected IOVA mode 'PA'
>>> VHOST_PORT: Failed to get VMDq info.
>>> EAL: Error - exiting with code: 1
>>>     Cause: Cannot initialize network ports
>>>
>>> The offending code is from examples/vhost/main.c:
>>>
>>>           if (dev_info.max_vmdq_pools == 0) {
>>>                   RTE_LOG(ERR, VHOST_PORT, "Failed to get VMDq info.\n");
>>>                   return -1;
>>>           }
>>>
>>> This is because the TAP PMD doesn't support VMDq pools.
>>>
>>> Is there an easy way to get this to work?
>>
>> This sounds strange to require VMDq support...
>> Copying Maxime and Chenbo who probably know better about this example code.
>>
>> Alternatively, did you consider using testpmd with the vhost pmd instead ?
>>
>>
> 
> Maybe you could use testpmd application instead, with net_tap PMD and a net_pcap PMD?
> 
> An alternative to net_tap could be to use Virtio-user PMD with Vhost- kernel backend.
> 
> Maxime
>