Re: [edk2-devel] [PATCH 2/2] MdeModulePkg/DxeCore: Fix stack overflow issue due to nested interrupts

["Paolo Bonzini" <pbonzini@redhat.com>]

On 2/29/24 20:22, Michael Brown wrote:
>> The design of NestedInterruptTplLib is that each nested interrupt must increase the TPL, but if I understand correctly there is a hole here:
>>
>>    //
>>    // Call RestoreTPL() to allow event notifications to be
>>    // dispatched.  This will implicitly re-enable interrupts.
>>    //
>>    gBS->RestoreTPL (InterruptedTPL);
>>
>>    //
>>    // Re-disable interrupts after the call to RestoreTPL() to ensure
>>    // that we have exclusive access to the shared state.
>>    //
>>    DisableInterrupts ();
>>
>> because gBS->RestoreTPL will unconditionally enable interrupts if InterruptedTPL < TPL_HIGH_LEVEL.
> 
> There's no hole there.

Yes, what I meant is that the whole of NestedInterruptTplLib is designed 
around plugging that hole.  But if you can avoid re-enabling interrupts 
at the end of CoreRestoreTpl(), you don't need DisableInterruptsOnIret() 
anymore.

> See the comments in the code for further details - I made them fairly 
> extensive.  🙂

Yup, I remember. :)

>> If possible, the easiest solution would be to merge 
>> NestedInterruptTplLib into Core DXE.
> 
> The question with that approach would be how to cleanly violate the 
> abstraction layer that separates the timer interrupt handler (existing 
> in a separate DXE driver executable) from the implementation of 
> CoreRestoreTplInternal() (existing in core DXE and not exposed via the 
> boot services table).

See outline in the other mail I have sent.

Paolo



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