Documentation/tracing: Mention that RESET_ATTACK_MITIGATION can clear memory

At the 2024 Linux Plumbers Conference, I was talking with Hans de Goede
about the persistent buffer to display traces from previous boots. He
mentioned that UEFI can clear memory. In my own tests I have not seen
this. He later informed me that it requires the config option:

 CONFIG_RESET_ATTACK_MITIGATION

It appears that setting this will allow the memory to be cleared on boot
up, which will definitely clear out the trace of the previous boot.

Add this information under the trace_instance in kernel-parameters.txt
to let people know that this can cause issues.

Link: https://lore.kernel.org/all/20170825155019.6740-2-ard.biesheuvel@linaro.org/



Reported-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241007131653.35837081@gandalf.local.home

Documentation/admin-guide/kernel-parameters.txt

@@ -6867,6 +6867,12 @@
 
 				reserve_mem=12M:4096:trace trace_instance=boot_map^traceoff^traceprintk@trace,sched,irq
 
+			Note, saving the trace buffer across reboots does require that the system
+			is set up to not wipe memory. For instance, CONFIG_RESET_ATTACK_MITIGATION
+			can force a memory reset on boot which will clear any trace that was stored.
+			This is just one of many ways that can clear memory. Make sure your system
+			keeps the content of memory across reboots before relying on this option.
+
 			See also Documentation/trace/debugging.rst