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From: Kumar Gala <galak@kernel.crashing.org>
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Subject: Re: [PATCH] [POWERPC] Rework EXC_LEVEL_EXCEPTION_PROLOG code
Date: Thu, 1 May 2008 08:22:08 -0500
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Cc: linuxppc-dev@ozlabs.org, Paul Mackerras <paulus@samba.org>
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>> So we have 4 actual exceptions:
>> * CriticalInput (some external device signaled this.  There are two
>> concepts of critical.  One is error the other is high priority)
>> However this would have the same caveats as any ExternalInput  
>> handler.
>
> No, it's worse. It can interrupt code that normally has
> local_irq_disabled() and thus doesn't expect to be interrupted. That
> means that everything becomes unsafe including locks etc....

Fair.  Should local_irq_disable() clear MSR_SE & MSR_BE on classic  
parts?

> Note that driver that want to make active use of that probably want  
> some
> explicit local_crit_irq_disable/enable functions to be able to  
> implement
> some sort of synchronization.

Or we could just have local_irq_disable -- clear MSR_EE and MSR_CE

>> * Watchdog - pretty severe if this fires.
>>
>> * Debug - user space debug is pretty straight forward.  However we
>> have features like kprobes that require kernel level support.
>
> Which means we have to be extra careful, in fact, I consider it a  
> design
> bug of BookE to have made debug be a critical interrupt...

I consider the whole BookE debug arch a design bug :)

- k