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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

Hi,
I totally agree with Avi's comments.
This topic is really important for the future of DPDK.
So I think we must give some time to continue the discussion
and have netdev involved in the choices done.
As a consequence, these series should not be merged in the release 16.04.
Thanks for continuing the work.


2016-02-29 12:58, Avi Kivity:
> 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.

+1

> > 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).

+1
 
> > 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?

+1
 
> 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.

+1

> >>> 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.