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 14:02, Ray Ni wrote:
> In the end, it will lower the TPL to TPL_APPLICATION with interrupt enabled.
> 
> However, it's possible that another timer interrupt happens just in the end
> of RestoreTPL() function when TPL is TPL_APPLICATION.

How do non-OVMF platforms solve the issue?  Do they just have the same 
bug as in https://bugzilla.tianocore.org/show_bug.cgi?id=4162 ?

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.


If possible, the easiest solution would be to merge 
NestedInterruptTplLib into Core DXE.  This way, instead of calling 
gBS->RestoreTPL, NestedInterruptTplLib can call a custom version of 
CoreRestoreTpl that exits with interrupts disabled.  That is, something like

VOID EFIAPI CoreRestoreTplInternal(IN EFI_TPL NewTpl,
                                    IN BOOLEAN InterruptState)
{
   //
   // The caller can request disabled interrupts to access shared
   // state, but TPL_HIGH_LEVEL must *not* have them enabled.
   //
   ASSERT(!(NewTpl == TPL_HIGH_LEVEL && InterruptState));

   // ...

   gEfiCurrentTpl = NewTpl;
   CoreSetInterruptState (InterruptState);
}

Now, CoreRestoreTpl is just

   //
   // If lowering below HIGH_LEVEL, make sure
   // interrupts are enabled
   //
   CoreRestoreTplInternal(NewTpl, NewTpl < TPL_HIGH_LEVEL);

whereas NestedInterruptRestoreTPL can do

   //
   // Call RestoreTPL() to allow event notifications to be
   // dispatched.  This will implicitly re-enable interrupts,
   // but only if events have to be dispatched.
   //
   CoreRestoreTplInternal(InterruptedTPL, FALSE);

   //
   // Interrupts are now disabled, so we can access shared state.
   //

This avoids the unlimited nesting of interrupts because each stack frame 
will indeed have a higher TPL than the outer version.

Paolo



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