Re: [PATCH] watchdog: hpwdt: Remove spinlock acquire and BIOS calls from NMI context

Ingo Molnar
2018-02-14T10:39:25Z

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Feb 14, 2018 at 10:31:59AM +0100, Ingo Molnar wrote:
> > Because in this particular case it does not appear to be so: the reason for the
> > BIOS/firmware call appears to be to determine how we nmi_panic() after receiving
> > an NMI that no other NMI handler handled: with a passive-aggressive "I don't know"
> > panic message or with a slightly more informative panic message.
>
> However much I like just ripping all that out, I think the ROM call
> actually does that logging, or that is how I read things.
>
> If you look at the original Changelog for that driver:
>
> Hp is providing a Hardware WatchDog Timer driver that will only work with the
> specific HW Timer located in the HP ProLiant iLO 2 ASIC. The iLO 2 HW Timer
> will generate a Non-maskable Interrupt (NMI) 9 seconds before physically
> resetting the server, by removing power, so that the event can be logged to
> the HP Integrated Management Log (IML), a Non-Volatile Random Access Memory
> (NVRAM). The logging of the event is performed using the HP ProLiant ROM via
> an Industry Standard access known as a BIOS Service Directory Entry.

Ok, that appears to be the case, too bad.

But the good news: if this callback is executed only once per system lifetime then
we don't actually have to perform *any* modification on this driver, right? The
reason is that this callback will panic unconditionally after performing the BIOS
call. The control flow to the panic is unconditional:

spin_lock_irqsave(&rom_lock, rom_pl);
if (!die_nmi_called && !is_icru && !is_uefi)
asminline_call(&cmn_regs, cru_rom_addr);

...

if (!is_icru && !is_uefi) {
if (cmn_regs.u1.ral == 0) {
nmi_panic(regs, "An NMI occurred, but unable to determine source.\n");
...

nmi_panic(regs, "An NMI occurred. Depending on your system the reason "
"for the NMI is logged in any one of the following "
"resources:\n"
"1. Integrated Management Log (IML)\n"
"2. OA Syslog\n"
"3. OA Forward Progress Log\n"
"4. iLO Event Log");

This callback does not get executed when we get perf NMIs, correct?

Ingo