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 09:51:33 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Don Zickus <dzickus@...hat.com>
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 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.

    J
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