Re: [PATCH] powerpc/xmon: add debugfs entry for xmon

On Mon, 13 Feb 2017 19:00:42 -0200
"Guilherme G. Piccoli" [off-list ref] wrote:

Currently the xmon debugger is set only via kernel boot command-line.
It's disabled by default, and can be enabled with "xmon=on" on the
command-line. Also, xmon may be accessed via sysrq mechanism, but once
we enter xmon via sysrq,  it's  kept enabled until system is rebooted,
even if we exit the debugger. A kernel crash will then lead to xmon
instance, instead of triggering a kdump procedure (if configured), for
example.

This patch introduces a debugfs entry for xmon, allowing user to query
its current state and change it if desired. Basically, the "xmon" file
to read from/write to is under the debugfs mount point, on powerpc
directory. Reading this file will provide the current state of the
debugger, one of the following: "on", "off", "early" or "nobt". Writing
one of these states to the file will take immediate effect on the debugger.

Signed-off-by: Guilherme G. Piccoli <redacted>
---
* I had this patch partially done for some time, and after a discussion
at the kernel slack channel latest week, I decided to rebase and fix
some remaining bugs. I'd change 'x' option to always disable the debugger,
since with this patch we can always re-enable xmon, but today I noticed
Pan's patch on the mailing list, so perhaps his approach of adding a flag
to 'x' option is preferable. I can change this in a V2, if requested.
Thanks in advance!

xmon state changing after the first sysrq+x violates principle of least
astonishment, so I think that should be fixed.

Then the question is, is it worth making it runtime configurable with xmon
command or debugfs tunables?

Thanks,
Nick