linux-kernel - Re: [V6][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs

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Date:	Wed, 28 Sep 2011 13:44:00 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Robert Richter <robert.richter@....com>,
	"x86@...nel.org" <x86@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"ying.huang@...el.com" <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"avi@...hat.com" <avi@...hat.com>
Subject: Re: [V6][PATCH 4/6] x86, nmi:  add in logic to handle multiple
 events and unknown NMIs

On Wed, Sep 28, 2011 at 09:51:33AM -0700, Jeremy Fitzhardinge wrote:
> On 09/28/2011 05:37 AM, Don Zickus wrote:
> > On Wed, Sep 28, 2011 at 12:31:40PM +0200, Robert Richter wrote:
> >> On 23.09.11 15:17:13, Don Zickus wrote:
> >>> @@ -89,6 +89,15 @@ 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;
> >> I don't think we can leave this in. As said, there are cases that 2
> >> nmis trigger but the handler is called only once. Only the first would
> >> be handled then, and the second get lost cause there is no 2nd nmi
> >> call.
> > Right.  Avi, Jeremy what was your objection that needed this optimization
> > in the first place?
> 
> My only interest in the NMI code is its use of spinlocks, which seem
> inappropriate.

Right.  But I thought this was going to clash with your ticketed spinlock
stuff?

Cheers,
Don
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