[PATCH net] Documentation: networking: Add a caveat to nexthop_compat_mode sysctl

[PATCH net] Documentation: networking: Add a caveat to nexthop_compat_mode sysctl

[Petr Machata]

net.ipv4.nexthop_compat_mode was added when nexthop objects were added to
provide the view of nexthop objects through the usual lens of the route
UAPI. As nexthop objects evolved, the information provided through this
lens became incomplete. For example, details of resilient nexthop groups
are obviously omitted.

Now that 16-bit nexthop group weights are a thing, the 8-bit UAPI cannot
convey the >8-bit weight accurately. Instead of inventing workarounds for
an obsolete interface, just document the expectations of inaccuracy.

Fixes: b72a6a7ab957 ("net: nexthop: Increase weight to u16")
Signed-off-by: Petr Machata <petrm@nvidia.com>
---
 Documentation/networking/ip-sysctl.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index eacf8983e230..dcbb6f6caf6d 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -2170,6 +2170,12 @@ nexthop_compat_mode - BOOLEAN
 	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.
+
+	Note that as a backward-compatible mode, dumping of modern features
+	might be incomplete or wrong. For example, resilient groups will not be
+	shown as such, but rather as just a list of next hops. Also weights that
+	do not fit into 8 bits will show incorrectly.
+
 	Default: true (backward compat mode)
 
 fib_notify_on_flag_change - INTEGER
-- 
2.47.0

Re: [PATCH net] Documentation: networking: Add a caveat to nexthop_compat_mode sysctl

[Ido Schimmel]

On Mon, Dec 09, 2024 at 12:05:31PM +0100, Petr Machata wrote:
> net.ipv4.nexthop_compat_mode was added when nexthop objects were added to
> provide the view of nexthop objects through the usual lens of the route
> UAPI. As nexthop objects evolved, the information provided through this
> lens became incomplete. For example, details of resilient nexthop groups
> are obviously omitted.
> 
> Now that 16-bit nexthop group weights are a thing, the 8-bit UAPI cannot
> convey the >8-bit weight accurately. Instead of inventing workarounds for
> an obsolete interface, just document the expectations of inaccuracy.
> 
> Fixes: b72a6a7ab957 ("net: nexthop: Increase weight to u16")
> Signed-off-by: Petr Machata <petrm@nvidia.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>

Re: [PATCH net] Documentation: networking: Add a caveat to nexthop_compat_mode sysctl

[David Ahern]

On 12/9/24 4:05 AM, Petr Machata wrote:
> net.ipv4.nexthop_compat_mode was added when nexthop objects were added to
> provide the view of nexthop objects through the usual lens of the route
> UAPI. As nexthop objects evolved, the information provided through this
> lens became incomplete. For example, details of resilient nexthop groups
> are obviously omitted.
> 
> Now that 16-bit nexthop group weights are a thing, the 8-bit UAPI cannot
> convey the >8-bit weight accurately. Instead of inventing workarounds for
> an obsolete interface, just document the expectations of inaccuracy.
> 
> Fixes: b72a6a7ab957 ("net: nexthop: Increase weight to u16")
> Signed-off-by: Petr Machata <petrm@nvidia.com>
> ---
>  Documentation/networking/ip-sysctl.rst | 6 ++++++
>  1 file changed, 6 insertions(+)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>

Re: [PATCH net] Documentation: networking: Add a caveat to nexthop_compat_mode sysctl

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon, 9 Dec 2024 12:05:31 +0100 you wrote:
> net.ipv4.nexthop_compat_mode was added when nexthop objects were added to
> provide the view of nexthop objects through the usual lens of the route
> UAPI. As nexthop objects evolved, the information provided through this
> lens became incomplete. For example, details of resilient nexthop groups
> are obviously omitted.
> 
> Now that 16-bit nexthop group weights are a thing, the 8-bit UAPI cannot
> convey the >8-bit weight accurately. Instead of inventing workarounds for
> an obsolete interface, just document the expectations of inaccuracy.
> 
> [...]

Here is the summary with links:
  - [net] Documentation: networking: Add a caveat to nexthop_compat_mode sysctl
    https://git.kernel.org/netdev/net/c/bbe4b41259a3

You are awesome, thank you!
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