netdev - Re: [PATCH net-next v3 2/3] net: ipv4: add sysctl for nexthop api compatibility mode

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Date:   Mon, 27 Apr 2020 07:12:12 -0600
From:   David Ahern <dsahern@...il.com>
To:     Roopa Prabhu <roopa@...ulusnetworks.com>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, rdunlap@...radead.org,
        nikolay@...ulusnetworks.com, bpoirier@...ulusnetworks.com
Subject: Re: [PATCH net-next v3 2/3] net: ipv4: add sysctl for nexthop api
 compatibility mode

On 4/26/20 9:41 PM, Roopa Prabhu wrote:
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index 6fcfd31..02029b5 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -1553,6 +1553,20 @@ skip_notify_on_dev_down - BOOLEAN
>  	on userspace caches to track link events and evict routes.
>  	Default: false (generate message)
>  
> +nexthop_compat_mode - BOOLEAN
> +	Controls whether new route nexthop API is backward compatible
> +	with old route API. By default Route nexthop API maintains
> +	user space compatibility with old route API: Which means
> +	Route dumps and netlink notifications include both new and
> +	old route attributes. In systems which have moved to the new API,
> +	this compatibility mode provides a way to turn off the old
> +	notifications and route attributes in dumps. This sysctl is on
> +	by default but provides the ability to turn off compatibility
> +	mode allowing systems to run entirely with the new routing
> +	nexthop API. Old route API behaviour and support is not modified
> +	by this sysctl
> +	Default: true (backward compat mode)
> +

That description is a bit confusing, to me at least. It would be better
to state what changes happen when the sysctl is disabled. Something like:

New nexthop API provides a means for managing nexthops independent of
prefixes. Backwards compatibilty with old route format is enabled by
default which means route dumps and notifications contain the new
nexthop attribute but also the full, expanded nexthop definition.
Further, updates or deletes of a nexthop configuration generate route
notifications for each fib entry using the nexthop. Once a system
understands the new API, this sysctl can be disabled to achieve full
performance benefits of the new API by disabling the nexthop expansion
and extraneous notifications.