linux-kernel - Re: [RFC][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs

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Date:	Mon, 22 Aug 2011 11:25:23 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	x86@...nel.org, Andi Kleen <andi@...stfloor.org>,
	Robert Richter <robert.richter@....com>, ying.huang@...el.com,
	LKML <linux-kernel@...r.kernel.org>, jason.wessel@...driver.com
Subject: Re: [RFC][PATCH 4/6] x86, nmi:  add in logic to handle multiple
 events and unknown NMIs

On Mon, Aug 22, 2011 at 04:22:15PM +0200, Peter Zijlstra wrote:
> On Fri, 2011-08-19 at 16:37 -0400, Don Zickus wrote:
> > @@ -260,6 +260,8 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
> >         pr_emerg("Dazed and confused, but trying to continue\n");
> >  }
> >  
> > +DEFINE_PER_CPU(bool, swallow_nmi);
> > +
> >  static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
> >  {
> >         unsigned char reason = 0;
> > @@ -271,8 +273,28 @@ static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
> >          * NMI can not be detected/processed on other CPUs.
> >          */
> >         handled = nmi_handle(NMI_LOCAL, regs);
> > -       if (handled)
> > +       if (handled) {
> > +               /*
> > +                * When handling multiple NMI events, we are not
> > +                * sure if the second NMI was dropped (because of
> > +                * too many NMIs), piggy-backed on the same NMI
> > +                * (perf) or is queued right behind this NMI.
> > +                * In the last case, we may accidentally get an
> > +                * unknown NMI because the event is already handled.
> > +                * Flag for this condition and swallow it later.
> > +                *
> > +                * FIXME: This detection has holes in it mainly
> > +                * because we can't tell _when_ the next NMI comes
> > +                * in.  A multi-handled NMI event followed by an 
> > +                * unknown NMI a second later, clearly should not
> > +                * be swallowed.
> > +                */
> > +               if (handled > 1)
> > +                       __this_cpu_write(swallow_nmi, true);
> > +               else
> > +                       __this_cpu_write(swallow_nmi, false);
> >                 return;
> > +       }
> >  
> >         /* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
> >         raw_spin_lock(&nmi_reason_lock);
> > @@ -296,6 +318,8 @@ static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
> >         raw_spin_unlock(&nmi_reason_lock);
> >  
> >         unknown_nmi_error(reason, regs);
> > +
> > +       __this_cpu_write(swallow_nmi, false);
> >  } 
> 
> All writes, no reads... the actual dropping of NMIs got lost and now
> lives in patch 5 which purports to be about statistics only.

Oops.  I screwed up when breaking up the changes into multiple patches.
I'll fix that.  Thanks for catching that.

Cheers,
Don

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