[patch v2] manual: Update signal descriptions

[Florian Weimer]

* Stefan Liebler:

> On 2/3/25 11:52, Florian Weimer wrote:
>> * Stefan Liebler:
>> 
>>> On 1/21/25 11:14, Andreas Schwab wrote:
>>>> On Jan 21 2025, Florian Weimer wrote:
>>>>
>>>>> Stefan, does s390x call these notifications “ctrl-alt-del
>>>>> notifications”, or this there some other name for them?  Is this kernel
>>>>> code path really active on current systems?
>>>>
>>>> It is also available as function key 0x020c (Boot) in the vt keyboard
>>>> driver, and used by various other drivers in the kernel (look for
>>>> kill_cad_pid and ctrl_alt_del).
>>>>
>>>
>>> FYI: I've asked around in our kernel-team and the answer:
>>> Yes, the kernel sends SIGPWR on a machine check warning.
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/s390/kernel/nmi.c#n177
>>>
>>> Thus the description is fine.
>> 
>> Thanks.  Does this include the ctrl-alt-del reference?  I thought this
>> was about a different IBM thing. 8-)
>> 
>> Florian
>> 
> We don't exactly know what you mean, but we have:
> - kernel/reboot.c:822:void ctrl_alt_del(void)
> => The ctrl_alt_del implementation itself uses "kill_cad_pid(SIGINT,
> 1);" to send SIGINT to the init process.
>
> - arch/s390/kernel/nmi.c:192:			kill_cad_pid(SIGPWR, 1);
> => In case of a machine check warning, "kill_cad_pid(SIGPWR, 1);" is
> used to send SIGPWR to the init process (from the SIGPWR-description in
> the patch: "to the process designated to receive ctrl-alt-del
> notifications").
>
> - drivers/s390/char/sclp_quiesce.c:39:	ctrl_alt_del();
> => The signal quiesce is an s390x specific mechanism where HMC or
> zVM-hypervisor informs the running kernel that it shall shut down.
> Therefore "ctrl_alt_del();" is used which sends SIGINT to the init process.
>
> Thus from my perspective nmi.c is the one for the description of SIGPWR
> and the description is fine.
>
> If those information is not the desired one, please ask again with a few
> more hints about which ctrl-alt-del reference you mean.

Thank you for providing that background.  I didn't know how well the
terminology aligns with kernel sources.  I just thought it was a bit odd
that this very PC-ish terminology showed up in this particular context.

Thanks,
Florian