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Subject: Re: [RFC][PATCH 4/6] x86, nmi:  add in logic to handle multiple
 events and unknown NMIs
From: Peter Zijlstra <peterz@infradead.org>
To: Don Zickus <dzickus@redhat.com>
Cc: x86@kernel.org, Andi Kleen <andi@firstfloor.org>,
        Robert Richter <robert.richter@amd.com>, ying.huang@intel.com,
        LKML <linux-kernel@vger.kernel.org>, jason.wessel@windriver.com
Date: Mon, 22 Aug 2011 16:22:15 +0200
In-Reply-To: <1313786266-9585-5-git-send-email-dzickus@redhat.com>
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	 <1313786266-9585-5-git-send-email-dzickus@redhat.com>
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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.