xen: add support for automatic debug key actions in case of crash

xen: add support for automatic debug key actions in case of crash

When the host crashes it would sometimes be nice to have additional
debug data available which could be produced via debug keys, but
halting the server for manual intervention might be impossible due to
the need to reboot/kexec rather sooner than later.

Add support for automatic debug key actions in case of crashes which
can be activated via boot- or runtime-parameter.

Depending on the type of crash the desired data might be different, so
support different settings for the possible types of crashes.

The parameter is "crash-debug" with the following syntax:

  crash-debug-<type>=<string>

with <type> being one of:

  panic, hwdom, watchdog, kexeccmd, debugkey

and <string> a sequence of debug key characters with '+' having the
special semantics of a 10 millisecond pause.

So "crash-debug-watchdog=0+0qr" would result in special output in case
of watchdog triggered crash (dom0 state, 10 ms pause, dom0 state,
domain info, run queues).

Don't call key handlers in early boot, as some (e.g. for 'd') require
some initializations to be finished, like scheduler or idle domain.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>