Re: [edk2-devel] [PATCH v3 3/5] MdeModulePkg: Do nothing on NestedInterruptRestoreTPL(TPL_HIGH_LEVEL)

["Laszlo Ersek" <lersek@redhat.com>]

On 1/23/24 16:31, Michael Brown wrote:
> At TPL_HIGH_LEVEL, CPU interrupts are disabled (as per the UEFI
> specification) and so we should never encounter a situation in which
> an interrupt occurs at TPL_HIGH_LEVEL.
> 
> Restoring TPL to TPL_HIGH_LEVEL is always a no-op.  Return immediately
> from NestedInterruptRestoreTPL(TPL_HIGH_LEVEL), so that we do not need
> to consider the effect of this possible invariant violation on the
> remainder of the logic.
> 
> Signed-off-by: Michael Brown <mcb30@ipxe.org>
> ---
>  MdeModulePkg/Library/NestedInterruptTplLib/Tpl.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/MdeModulePkg/Library/NestedInterruptTplLib/Tpl.c b/MdeModulePkg/Library/NestedInterruptTplLib/Tpl.c
> index d56c12a44529..99af553ab189 100644
> --- a/MdeModulePkg/Library/NestedInterruptTplLib/Tpl.c
> +++ b/MdeModulePkg/Library/NestedInterruptTplLib/Tpl.c
> @@ -99,6 +99,19 @@ NestedInterruptRestoreTPL (
>    EFI_TPL  SavedInProgressRestoreTPL;
>    BOOLEAN  DeferredRestoreTPL;
>  
> +  //
> +  // At TPL_HIGH_LEVEL, CPU interrupts are disabled (as per the UEFI
> +  // specification) and so we should never encounter a situation in
> +  // which InterruptedTPL==TPL_HIGH_LEVEL.
> +  //
> +  // Restoring TPL to TPL_HIGH_LEVEL is always a no-op.  Return
> +  // immediately so that we do not need to consider the effect of this
> +  // possible invariant violation in the logic below.
> +  //
> +  if (InterruptedTPL >= TPL_HIGH_LEVEL) {
> +    return;
> +  }
> +
>    //
>    // If the TPL at which this interrupt occurred is equal to that of
>    // the in-progress RestoreTPL() for an outer instance of the same

Feels like the handling logic might as well be "panic" here (except edk2
does not have a central panic API that's suitable for all platforms). I
probably missed the previous discussion that led to this patch. Either
way, it seems reasonable.

Acked-by: Laszlo Ersek <lersek@redhat.com>



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