Merge branch 'add-notifications-when-route-hardware-flags-change'

Merge branch 'add-notifications-when-route-hardware-flags-change'

Ido Schimmel says:

====================
Add notifications when route hardware flags change

Routes installed to the kernel can be programmed to capable devices, in
which case they are marked with one of two flags. RTM_F_OFFLOAD for
routes that offload traffic from the kernel and RTM_F_TRAP for routes
that trap packets to the kernel for processing (e.g., host routes).

These flags are of interest to routing daemons since they would like to
delay advertisement of routes until they are installed in hardware. This
allows them to avoid packet loss or misrouted packets. Currently,
routing daemons do not receive any notifications when these flags are
changed, requiring them to poll the kernel tables for changes which is
inefficient.

This series addresses the issue by having the kernel emit RTM_NEWROUTE
notifications whenever these flags change. The behavior is controlled by
two sysctls (net.ipv4.fib_notify_on_flag_change and
net.ipv6.fib_notify_on_flag_change) that default to 0 (no
notifications).

Note that even if route installation in hardware is improved to be more
synchronous, these notifications are still of interest. For example, a
multipath route can change from RTM_F_OFFLOAD to RTM_F_TRAP if its
neighbours become invalid. A routing daemon can choose to withdraw /
replace the route in that case. In addition, the deletion of a route
from the kernel can prompt the installation of an identical route
(already in kernel, with an higher metric) to hardware.

For testing purposes, netdevsim is aligned to simulate a "real" driver
that programs routes to hardware.

Series overview:

Patches #1-#2 align netdevsim to perform route programming in a
non-atomic context

Patches #3-#5 add sysctl to control IPv4 notifications

Patches #6-#8 add sysctl to control IPv6 notifications

Patch #9 extends existing fib tests to set sysctls before running tests

Patch #10 adds test for fib notifications over netdevsim
====================

Link: https://lore.kernel.org/r/20210201194757.3463461-1-idosch@idosch.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>