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

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Date:   Wed, 14 Feb 2018 11:39:18 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>, Dave Hansen <dave@...1.net>,
        hpa@...or.com, tglx@...utronix.de, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, dwmw@...zon.co.uk,
        linux-tip-commits@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH] watchdog: hpwdt: Remove spinlock acquire and BIOS calls
 from NMI context


* Peter Zijlstra <peterz@...radead.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