linux-kernel - Re: [PATCH -v2 7/7] x86, NMI, Remove do_nmi

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Message-ID: <1285633705.20791.84.camel@yhuang-dev>
Date:	Tue, 28 Sep 2010 08:28:25 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Robert Richter <robert.richter@....com>,
	huang ying <huang.ying.caritas@...il.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH -v2 7/7] x86, NMI, Remove do_nmi_callback logic

Hi, Don,

On Mon, 2010-09-27 at 23:16 +0800, Don Zickus wrote:
> On Mon, Sep 27, 2010 at 03:43:41PM +0200, Robert Richter wrote:
> > On 27.09.10 08:56:44, huang ying wrote:
> > 
> > > >> -static int unknown_nmi_panic_callback(struct pt_regs *regs, int cpu)
> > > >> -{
> > > >> -     unsigned char reason = get_nmi_reason();
> > > >> -     char buf[64];
> > > >> -
> > > >> -     sprintf(buf, "NMI received for unknown reason %02x\n", reason);
> > > >> -     die_nmi(buf, regs, 1); /* Always panic here */
> > > >> -     return 0;
> > > >
> > > > You are dropping this code that is different to panic().
> > > 
> > > What is the difference? Is it relevant?
> > 
> > I think yes, since the code behaves different. Otherwise we could
> > remove die_nmi() completly and replace it by panic(). But both are
> > different implementions. Maybe we can merge the code, but I didn't
> > look at it closly.
> 
> Actually die_nmi is a wrapper around panic with two important pieces.
> One, it dumps some registers and two it does another notifier call to
> DIE_NMIWATCHDOG (which correlates to another discussion in this patch
> series).
> 
> So if we do any consolidation between panic and die_nmi, it should be
> convert to die_nmi.  But then I wonder if that breaks the original
> semantics of 'panic_on_unrecovered_nmi'.  I don't think so though.

Please take a look at the original code:


                if (nmi_watchdog_tick(regs, reason))
                        return;
                if (!do_nmi_callback(regs, cpu))
#endif /* !CONFIG_LOCKUP_DETECTOR */
                        unknown_nmi_error(reason, regs);
#else
                unknown_nmi_error(reason, regs);
#endif

If NMI comes from watchdog, nmi_watchdog_tick() will return 1. So
do_nmi_callback() is NOT for watchdog NMI, but for unknown NMI. Why do
we call DIE_NMIWATCHDOG for unknown NMI (NOT watchdog NMI)? die_nmi is
for watchdog, not unknown NMI.

So another issue is registers dumping. If it is necessary, we can dump
registers in unknown_nmi_error(). Although I think unknown NMI comes
from hardware instead of software, so it does not help much to dump
registers.

Best Regards,
Huang Ying


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