[PATCH v5] doc: note KNI alternatives

[Ferruh Yigit]

Add more information on alternatives of KNI and the disadvantages of KNI
against these alternatives.

Signed-off-by: Ferruh Yigit <ferruh.yigit at amd.com>
---
Cc: Olivier Matz <olivier.matz at 6wind.com>
Cc: David Marchand <david.marchand at redhat.com>
Cc: 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>
Cc: Morten Brørup <mb at smartsharesystems.com>

v5:
* Rebased
* Dedicated kernel core added as disadvantage
---
 .../prog_guide/kernel_nic_interface.rst       | 35 ++++++++++++++++---
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/doc/guides/prog_guide/kernel_nic_interface.rst b/doc/guides/prog_guide/kernel_nic_interface.rst
index 6a564f61cad8..cac0ab0cb278 100644
--- a/doc/guides/prog_guide/kernel_nic_interface.rst
+++ b/doc/guides/prog_guide/kernel_nic_interface.rst
@@ -11,8 +11,9 @@ Kernel NIC Interface
    KNI is deprecated and will be removed in future.
    See :doc:`../rel_notes/deprecation`.
 
-   For an alternative to KNI, that does not require any out-of-tree Linux kernel modules,
-   or a custom library, see :ref:`virtio_user_as_exception_path`.
+   :ref:`virtio_user_as_exception_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.
 
 .. note::
 
@@ -21,14 +22,38 @@ Kernel NIC Interface
 
 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_exception_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 disadvantages of the KNI are:
+
+* 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.
+
+* Requires dedicated kernel cores.
 
-*   Allows an interface with the kernel network stack.
+* Only a subset of net devices control commands are supported by KNI.
 
 The components of an application using the DPDK Kernel NIC Interface are shown in :numref:`figure_kernel_nic_intf`.
 
-- 
2.25.1