Re: [V5][PATCH 4/6] x86, nmi: add in logic to handle multipleevents and unknown NMIs

[Robert Richter]

On 21.09.11 11:33:52, Peter Zijlstra wrote:
> 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.

No, I am trying to explain this...

If hw triggers more than one nmi *before* the nmi handler is entered
(no matter what the source is), then there will be no back-to-back
nmi. NMIs are edge triggered and there is only one pending signal (AMD
cpus). The signal is cleared when entering the handler. If the cpu
executes microcode or smm code and delays execution of the nmi
handler, then there is no second call of the handler even if there are
2 sources for it.

-Robert


-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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