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

["Michael Brown" <mcb30@ipxe.org>]

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
-- 
2.43.0



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