Re: [dpdk-dev] [PATCH 1/3] kcp: add kernel control path kernel module

[Avi Kivity <avi@scylladb.com>]

On 02/29/2016 12:43 PM, Ferruh Yigit wrote:
> On 2/29/2016 9:43 AM, Avi Kivity wrote:
>> On 02/28/2016 10:16 PM, Ferruh Yigit wrote:
>>> On 2/28/2016 3:34 PM, Avi Kivity wrote:
>>>> On 01/27/2016 06:24 PM, Ferruh Yigit wrote:
>>>>> This kernel module is based on KNI module, but this one is stripped
>>>>> version of it and only for control messages, no data transfer
>>>>> functionality provided.
>>>>>
>>>>> This Linux kernel module helps userspace application create virtual
>>>>> interfaces and when a control command issued into that virtual
>>>>> interface, module pushes the command to the userspace and gets the
>>>>> response back for the caller application.
>>>>>
>>>>> The Linux tools like ethtool/ifconfig/ip can be used on virtual
>>>>> interfaces but not ones for related data, like tcpdump.
>>>>>
>>>>> In long term this patch intends to replace the KNI and KNI will be
>>>>> depreciated.
>>>> Instead of adding yet another out-of-tree kernel module, why not extend
>>>> the existing in-tree tap driver?  This will make everyone's life easier.
>>>>
>>>> Since tap also supports data transfer, an application can also forward
>>>> packets not intended to it to the kernel, and forward packets from the
>>>> kernel through the device.
>>>>
>>> Hi Avi,
>>>
>>> KDP (Kernel Data Path) does what you have described, it is implemented
>>> as PMD and it benefits from tap driver to data transfer through the
>>> kernel. It also support custom kernel module for better performance.
>>>
>>> For KCP (Kernel Control Path), network driver forwards control commands
>>> to the userspace driver, I doubt this is something wanted for tun/tap
>>> driver, so extending tun/tap driver like this can be hard to upstream.
>> Have you tried asking?  Maybe if you explain it they will be open to the
>> extension.
>>
> Not communicated but tun/tap already doing something different.
> For KCP, created interface is map of the DPDK port. All data interface
> shows coming from DPDK port. For example if you get stats information
> with ifconfig, the values you observe are DPDK port statistics -not
> statistics of data between userspace and kernelspace, statistics of data
> forwarded between DPDK ports. If you down the interface, DPDK port
> stopped, etc...
>
> If you extend the tun/tap, it won't be map of the DPDK port, and if you
> get statistics information from that interface, what do you expect to
> see, the data transferred between kernel and userspace, or underlying
> DPDK port forwarding statistics?

Good point.  But you really have to involve netdev on this, or you'll 
live out-of-tree forever.

> Extending tun/tap in a way we want, forwarding all control commands to
> userspace, will break the current tun/tap, this doesn't looks like a
> valid option to me.

It's possible to enhance it while preserving backwards compatibility, by 
enabling a feature flag (statistics from userspace).

> For data path, using tun/tap is OK and we are already doing it, for the
> control path I believe we need a new driver.
>
>> Certainly it will be better to have KCP and KDP use the same kernel
>> interface name; so we'll need to either add data path support to kcp
>> (causing duplication with tap), or add control path support to tap. I
>> think the latter is preferable.
>>
> Why it is better to have same interface? Anyone who is not interested
> with kernel data path may want to control DPDK ports using common tools,
> or want to get some basic information and stats using ethtool or
> ifconfig. Why we need to bind two different functionality together?

Having two interfaces will be confusing for the user.  If I wish to 
firewall data packets coming from the dpdk port, do I set firewall rules 
on dpdk0 or tap0?

I don't think it matters whether you extend tap, or add a data path to 
kcp, but if you want to upstream it, it needs to be blessed by netdev.

>
>>> We are investigating about adding a native support to Linux kernel for
>>> KCP, but there is no task started for this right now, any support is
>>> welcome.
>>>
>>>