linux-kernel - Re: [V5][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs

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Date:	Wed, 21 Sep 2011 13:43:57 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Don Zickus <dzickus@...hat.com>
CC:	"x86@...nel.org" <x86@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Robert Richter <robert.richter@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"avi@...hat.com" <avi@...hat.com>,
	"jeremy@...p.org" <jeremy@...p.org>
Subject: Re: [V5][PATCH 4/6] x86, nmi:  add in logic to handle multiple events
 and unknown NMIs

On 09/20/2011 10:43 PM, Don Zickus wrote:
[snip]
> @@ -313,7 +359,31 @@ static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
>  	}
>  	raw_spin_unlock(&nmi_reason_lock);
>  
> -	unknown_nmi_error(reason, regs);
> +	/*
> +	 * Only one NMI can be latched at a time.  To handle
> +	 * this we may process multiple nmi handlers at once to
> +	 * cover the case where an NMI is dropped.  The downside
> +	 * to this approach is we may process an NMI prematurely,
> +	 * while its real NMI is sitting latched.  This will cause
> +	 * an unknown NMI on the next run of the NMI processing.
> +	 * 
> +	 * We tried to flag that condition above, by setting the
> +	 * swallow_nmi flag when we process more than one event.
> +	 * This condition is also only present on the second half
> +	 * of a back-to-back NMI, so we flag that condition too.
> +	 *
> +	 * If both are true, we assume we already processed this
> +	 * NMI previously and we swallow it.  Otherwise we reset
> +	 * the logic.
> +	 *
> +	 * I am sure there are scenarios where we accidentally
> +	 * swallow a real 'unknown' NMI.  But this is the best
> +	 * we can do for now.

Why not describe a scenario where we swallow a real 'unknown' NMI?  So
that someone working on the code in the future will know the challenge?

Best Regards,
Huang Ying

> +	 */
> +	if (b2b && __this_cpu_read(swallow_nmi))
> +		;
> +	else
> +		unknown_nmi_error(reason, regs);
>  }
>  
>  dotraplinkage notrace __kprobes void

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