Re: [dpdk-dev] [PATCH v2 10/10] kni: add API to set link status on kernel interface

[Dan Gora <dg@adax.com>]

On Wed, Aug 29, 2018 at 12:54 PM, Stephen Hemminger
<stephen@networkplumber.org> wrote:
> On Thu, 28 Jun 2018 18:55:08 -0700
> Dan Gora <dg@adax.com> wrote:
>
>> Add a new API function to KNI, rte_kni_update_link() to allow DPDK
>> applications to update the link state for the KNI network interfaces
>> in the linux kernel.
>>
>> Note that the default carrier state is set to off when the interface
>> is opened.
>>
>> Signed-off-by: Dan Gora <dg@adax.com>
>
> Do you really need a special ioctl for this?
> There is already ability to set link state via sysfs or netlink.

I think yes.. AFAIK sysfs does not constitute a stable API; it's only
available for Linux (yes, I know KNI is linux-only currently, but
there's not really any technical reason why it can't work on BSD) and
there are already callbacks to change the MTU and MAC addresses which
could also be done via netlink.  IMHO having the kernel have an
accurate view of the link state is more important than the ability to
change the MAC address of the interface...

In our application we want the linux kernel/"normal" userspace to be
able to use the DPDK controlled interfaces like any other interface.
We need to be able to assign IP addresses to them, have them
participate in routing, etc.  Since they are controlled via our DPDK
application, there is no way for the kernel to know when the cable is
connected/removed since that information is only communicated to the
DPDK application.

The other option, which I toyed with but decided against, would be to
have a polling thread in the KNI module to call a callback into the
DPDK application to poll the link status.  However that would still
possibly leave a time period when the link is down, but the kernel
does not know about it.  I decided that it would probably be best to
just have a way for the DPDK application to inform the linux kernel
(via the KNI module) that the link was down.

It's important for the linux kernel to know about the link status if
the interface is going to be treated like any other.  Things like
assigning IP addresses and adding the interfaces to the routing table
happen automatically when the link is marked "up".  If the link is not
marked "up", or is "up" when it should be "down", then the kernel
cannot configure that interface correctly, or will use it when it
should not be.

thanks
dan