Add documentation explaining the msgid feature in netconsole.

This feature appends unique id to the userdata dictionary. The message
ID is populated from a per-target 32 bit counter which is incremented
for each message sent to the target. This allows a target to detect if
messages are dropped before reaching the target.

Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
 Documentation/networking/netconsole.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Documentation/networking/netconsole.rst b/Documentation/networking/netconsole.rst
index a0076b542e9c..42a0acf2eb5e 100644
--- a/Documentation/networking/netconsole.rst
+++ b/Documentation/networking/netconsole.rst
@@ -340,6 +340,28 @@ In this example, the message was sent by CPU 42.
       cpu=42    # kernel-populated value
 
 
+Message ID auto population in userdata
+--------------------------------------
+
+Within the netconsole configfs hierarchy, there is a file named `msgid_enabled`
+located in the `userdata` directory. This file controls the message ID
+auto-population feature, which assigns a unique id to each message sent to a
+given target and appends the ID to userdata dictionary in every message sent.
+
+The message ID is built from a per-target 32 bit counter that is incremented
+for every message sent to the target. This ID can be used by the target to
+detect if messages were dropped before reaching the target.
+
+Example::
+
+  echo "This is message #1" > /dev/kmsg
+  echo "This is message #2" > /dev/kmsg
+  13,434,54928466,-;This is message #1
+   msgid=1
+  13,435,54934019,-;This is message #2
+   msgid=2
+
+
 Extended console:
 =================
 

-- 
2.47.1

On Wed, Jun 11, 2025 at 07:36:07AM -0700, Gustavo Luiz Duarte wrote:
> Add documentation explaining the msgid feature in netconsole.
> 
> This feature appends unique id to the userdata dictionary. The message
> ID is populated from a per-target 32 bit counter which is incremented
> for each message sent to the target. This allows a target to detect if
> messages are dropped before reaching the target.
> 
> Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
> ---
>  Documentation/networking/netconsole.rst | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/Documentation/networking/netconsole.rst b/Documentation/networking/netconsole.rst
> index a0076b542e9c..42a0acf2eb5e 100644
> --- a/Documentation/networking/netconsole.rst
> +++ b/Documentation/networking/netconsole.rst
> @@ -340,6 +340,28 @@ In this example, the message was sent by CPU 42.
>        cpu=42    # kernel-populated value
>  
>  
> +Message ID auto population in userdata
> +--------------------------------------
> +
> +Within the netconsole configfs hierarchy, there is a file named `msgid_enabled`
> +located in the `userdata` directory. This file controls the message ID
> +auto-population feature, which assigns a unique id to each message sent to a

Important to say that the message id will eventually wrap, thus it is
not very unique.

> +given target and appends the ID to userdata dictionary in every message sent.
> +
> +The message ID is built from a per-target 32 bit counter that is incremented
> +for every message sent to the target. This ID can be used by the target to
> +detect if messages were dropped before reaching the target.

Please also add that we cannot rely on console ids, given some messages
never make netconsole, thus, we never now if the message was never sent,
or, never received.