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

[Ferruh Yigit <ferruh.yigit@intel.com>]

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?

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.

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?

>> 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.
>>
>>
>