Re: a3b2cb30 broken panic reporting for qemu guests

On Wed, 29 Nov 2017 15:06:52 +1100
David Gibson [off-list ref] wrote:

a3b2cb30 "powerpc: Do not call ppc_md.panic in fadump panic notifier"
purports to fix a problem when the kernel panics with fadump not
registered, but it breaks something else instead.  I _think_ it was
working on the incorrect assumption that ppc_md.panic was (or should
be) only used with fadump, but I'm not really sure.

Panic works with kdump enabled, and (I think) with fadump enabled).
However, with neither of these enabled, we always go to the generic
panic logic.

Yeah thanks, I can't remember what assumption I was working on tbh.
 

That's incorrect for PAPR guests - they should call ibm,os-term via
RTAS.  Under qemu this leads to a "GUEST_PANICKED" event notification
which higher-level management pays attention to.  Since a3b2cb30 we
now reboot instead of reporting that.

I believe it will also break panic for PS3 machines, but since that
platform basically no longer exists, we probably don't care.

I (hope) it should just go down to the normal panic path and not do
much worse than it already does -- although it won't print out that
message.

I'm not entirely sure how to fix this.  I _think_ what we want is to
call ppc_md.panic from a late panic notifier, the way this patch does
for fadump_panic_event() if fadump is registered.

The problem I had there is that some of the printk and console stuff
wasn't getting flushed out, so I was getting a blank screen. This was
probably in conjunction with panicing from NMI context that we're now
starting to introduce.

So it's a bit annoying. There's other ugliness we have for being unable
to control panic code well enough from arch code
(arch/powerpc/platforms/powernv/opal.c)

I guess a really minimal fix is to put an #ifdef powerpc down the bottom
there (/me *cries*).