[PATCH v3 1/2] doc: note KNI alternatives

[Morten Brørup]

> From: Ferruh Yigit [mailto:ferruh.yigit at intel.com]
> Sent: Wednesday, 24 November 2021 18.16
> 
> Add more information on alternatives of KNI and the cons of KNI against
> these alternatives.
> 
> Signed-off-by: Ferruh Yigit <ferruh.yigit at intel.com>
> ---
> Cc: Olivier Matz Olivier Matz <olivier.matz at 6wind.com>
> Cc: David Marchand David Marchand <david.marchand at redhat.com>
> Cc: Stephen Hemminger Stephen Hemminger <stephen at networkplumber.org>
> Cc: Elad Nachman <eladv6 at gmail.com>
> Cc: Igor Ryzhov <iryzhov at nfware.com>
> Cc: Dan Gora <dg at adax.com>
> 
> v3:
> * reference tap document directly instead of adding label to it.
> ---
>  .../prog_guide/kernel_nic_interface.rst       | 34 +++++++++++++++++--
>  1 file changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/doc/guides/prog_guide/kernel_nic_interface.rst
> b/doc/guides/prog_guide/kernel_nic_interface.rst
> index 1ce03ec1a374..f5a8b7c0782c 100644
> --- a/doc/guides/prog_guide/kernel_nic_interface.rst
> +++ b/doc/guides/prog_guide/kernel_nic_interface.rst
> @@ -6,16 +6,44 @@
>  Kernel NIC Interface
>  ====================
> 
> +.. Note::
> +
> +   :ref:`virtio_user_as_exceptional_path` alternative is the preferred
> way for
> +   interfacing with the Linux network stack as it is an in-kernel
> solution and
> +   has similar performance expectations.
> +
>  The DPDK Kernel NIC Interface (KNI) allows userspace applications
> access to the Linux* control plane.
> 
> -The benefits of using the DPDK KNI are:
> +KNI provides an interface with the kernel network stack and allows
> management of
> +DPDK ports using standard Linux net tools such as ``ethtool``,
> ``ifconfig`` and
> +``tcpdump``.
> +
> +The main use case of KNI is to get/receive exception packets from/to
> Linux network
> +stack while main datapath IO is done bypassing the networking stack.
> +
> +There are other alternatives to KNI, all are available in the upstream
> Linux:
> +
> +#. :ref:`virtio_user_as_exceptional_path`
> +
> +#. :doc:`../nics/tap` as wrapper to `Linux tun/tap
> +   <https://www.kernel.org/doc/Documentation/networking/tuntap.txt>`_
> +
> +The benefits of using the KNI against alternatives are:
> 
>  *   Faster than existing Linux TUN/TAP interfaces
>      (by eliminating system calls and copy_to_user()/copy_from_user()
> operations.
> 
> -*   Allows management of DPDK ports using standard Linux net tools
> such as ethtool, ifconfig and tcpdump.
> +The cons of the KNI are:

Consider if "disadvantages" is more appropriate than "cons". (I'm not a native English speaker.)

> +
> +* It is out-of-tree Linux kernel module and it can't be distributed as
> binary as
> +  part of operating system vendor DPDK packages. This makes it harder
> to
> +  consume, although it is always possible to compile it from the
> source code.
> +
> +* As it shares memory between userspace and kernelspace, and kernel
> part
> +  directly uses input provided by userspace, it is not safe. This
> makes hard to
> +  upstream the module.
> 
> -*   Allows an interface with the kernel network stack.
> +* Only a subset of net devices control commands are supported by KNI.

If it is still relevant, add something along the lines of:
* Requires dedicated kernel cores.

> 
>  The components of an application using the DPDK Kernel NIC Interface
> are shown in :numref:`figure_kernel_nic_intf`.
> 
> --
> 2.31.1
> 

Could you perhaps also promote the virtio-user documentation by moving it from the HowTo Guides to the Programmer's Guide?

-Morten