linux-kernel - Re: [V4][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs

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Date:	Wed, 14 Sep 2011 08:56:40 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	x86@...nel.org, Andi Kleen <andi@...stfloor.org>,
	Robert Richter <robert.richter@....com>,
	Peter Zijlstra <peterz@...radead.org>, ying.huang@...el.com
Cc:	LKML <linux-kernel@...r.kernel.org>, paulmck@...ux.vnet.ibm.com,
	avi@...hat.com, jeremy@...p.org
Subject: Re: [V4][PATCH 4/6] x86, nmi:  add in logic to handle multiple
 events and unknown NMIs

On Tue, Sep 13, 2011 at 04:58:27PM -0400, Don Zickus wrote:
> 
> V3:
>   - redesigned the algorithm to utilize Avi's idea of detecting a back-to-back
>     NMI with %rip.

Hi Robert,

I realized I added an optimization for executing the nmi handlers to help
minimize the impact on the virt folks and realize it might break your IBS
stuff.

> -static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs)
> +static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs, bool b2b)
>  {
>  	struct nmi_desc *desc = nmi_to_desc(type);
>  	struct nmiaction *next_a, *a, **ap = &desc->head;
> @@ -87,6 +87,16 @@ static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs)
>  
>  		handled += a->handler(type, regs);
>  
> +		/*
> + 		 * Optimization: only loop once if this is not a 
> + 		 * back-to-back NMI.  The idea is nothing is dropped
> + 		 * on the first NMI, only on the second of a back-to-back
> + 		 * NMI.  No need to waste cycles going through all the
> + 		 * handlers.
> + 		 */
> +		if (!b2b && handled)
> +			break;
> +
>  		a = next_a;
>  	}
>  	rcu_read_unlock();

The optimization is to run through the handlers until one of them claims
the NMI but only for the first NMI.  Whereas on the second half of a
back-to-back NMI, run through all the handlers regardless of how many
claim they handled it.

Does your IBS stuff need to always run through two handlers?

Cheers,
Don
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