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Re: [PATCH v3] xen: add support for automatic debug key actions in case of crash



On 11.12.20 08:24, Jan Beulich wrote:
On 11.12.2020 08:02, Jürgen Groß wrote:
On 10.12.20 21:51, Julien Grall wrote:
Hi Jan,

On 09/12/2020 14:29, Jan Beulich wrote:
On 09.12.2020 13:11, Julien Grall wrote:
On 26/11/2020 11:20, Jan Beulich wrote:
On 26.11.2020 09:03, Juergen Gross wrote:
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).

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V2:
- switched special character '.' to '+' (Jan Beulich)
- 10 ms instead of 1 s pause (Jan Beulich)
- added more text to the boot parameter description (Jan Beulich)

V3:
- added const (Jan Beulich)
- thorough test of crash reason parameter (Jan Beulich)
- kexeccmd case should depend on CONFIG_KEXEC (Jan Beulich)
- added dummy get_irq_regs() helper on Arm

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Except for the Arm aspect, where I'm not sure using
guest_cpu_user_regs() is correct in all cases,

I am not entirely sure to understand what get_irq_regs() is supposed to
returned on x86. Is it the registers saved from the most recent
exception?

An interrupt (not an exception) sets the underlying per-CPU
variable, such that interested parties will know the real
context is not guest or "normal" Xen code, but an IRQ.

Thanks for the explanation. I am a bit confused to why we need to give a
regs to handle_keypress() because no-one seems to use it. Do you have an
explanation?

dump_registers() (key 'd') is using it.


To add to the confusion, it looks like that get_irqs_regs() may return
NULL. So sometimes we may pass guest_cpu_regs() (which may contain
garbagge or a set too far).

I guess this is a best effort approach.

Indeed. If there are ways to make it "more best", we should of
course follow them. (Except before Dom0 starts, I'm afraid I
don't see though where garbage would come from. And even then,
just like for the idle vCPU-s, it shouldn't really be garbage,
or else this suggests missing initialization somewhere.)

I guess providing the wrong information to handle_keypress() is not
going to matter that much because no-one use it (?). Although, I'd like
to make sure this is not going to bite us in the future.

TBH using the 'd' handler isn't making that much sense, as the
information delivered would be of interest only in case of a panic(),
which is already printing that information.

I disagree. I've had numerous cases where I found this key very
useful. Or do you mean what you say just for the new purpose
added here?

Just for the new purpose.


Juergen

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