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 17:33:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Robert Richter <robert.richter@....com>
Cc:	Don Zickus <dzickus@...hat.com>, "x86@...nel.org" <x86@...nel.org>,
	Andi Kleen <andi@...stfloor.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>,
	"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 Wed, 2011-09-21 at 17:18 +0200, Robert Richter wrote:
> 1. The cpu executes some microcode or SMM code.
> 2. HW triggers the first NMI, an NMI is pending.
> 3. HW triggers a second NMI, the NMI is still pending.
> 4. The cpu finished microcode or SMM code.
> 5. NMI handler is called, no NMI pending anymore.
> 6. Return from NMI handler. 

Even without SMM, all you need is two different NMI users to trigger
while an NMI is in flight.

Wouldn't be entirely impossible to trigger if you have
non-fatal-MCE/hardware-NMI-switch/PMI all active.
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