Add extra RestoreTpl() call in DiskIo to maintain TPL raise/restore s…

…ymmetry (#230)

Adds a call to RestoreTpl() in DiskIo2ReadWriteDisk(). While the current
implementation does not technically violate spec on raise/restore TPL,
this extra call ensures symmetry between RaiseTpl and RestoreTpl calls,
which makes analysis of TPL correctness simpler and permits certain
non-standard TPL usages that some platforms require.

- [ ] Impacts functionality?
- [ ] Impacts security?
- [ ] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?

Booted a system using this driver for disk access, observed that
symmetry between Raise/Restore TPL was maintained as expected.

N/A

Remove unnecessary TPL restore call (#229)

Comments out a redundant call to RestoreTpl(). While this does not
technically violate spec on raise/restore TPL, TPL should already be at
the specified level. This extra call introduces an asymmetry between
RaiseTpl and RestoreTpl calls, which makes analysis of TPL correctness
more difficult and hampers certain non-standard TPL usages that some
platforms require.

- [ ] Impacts functionality?
- [ ] Impacts security?
- [ ] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?

Booted and observed no un-intended side effects w.r.t. TPL with this
modification. Added test instrumentation to verify that TPL is always
already at the desired state prior to this call being executed.

N/A

---------

Co-authored-by: Michael Kubacki <michael.kubacki@microsoft.com>

MdeModulePkg/Core/Dxe/Image/Image.c

@@ -1768,7 +1768,10 @@ CoreStartImage (
   // Image has completed.  Verify the tpl is the same
   //
   ASSERT (Image->Tpl == gEfiCurrentTpl);
-  CoreRestoreTpl (Image->Tpl);
+  if (Image->Tpl != gEfiCurrentTpl) {
+    // MU_CHANGE - reduce TPL restore
+    CoreRestoreTpl (Image->Tpl);
+  } // MU_CHANGE - reduce TPL restore
 
   CoreFreePool (Image->JumpBuffer);
 

MdeModulePkg/Universal/Disk/DiskIoDxe/DiskIo.c

@@ -847,6 +847,7 @@ DiskIo2ReadWriteDisk (
   DISK_IO_SUBTASK         *Subtask;
   DISK_IO2_TASK           *Task;
   EFI_TPL                 OldTpl;
+  EFI_TPL                 OldTpl1; // MU_CHANGE - TPL Symmetry
   BOOLEAN                 Blocking;
   BOOLEAN                 SubtaskBlocking;
   LIST_ENTRY              *SubtasksPtr;
@@ -977,7 +978,7 @@ DiskIo2ReadWriteDisk (
     }
   }
 
-  gBS->RaiseTPL (TPL_NOTIFY);
+  OldTpl1 = gBS->RaiseTPL (TPL_NOTIFY); // MU_CHANGE - TPL symmetry
 
   //
   // Remove all the remaining subtasks when failure.
@@ -1012,6 +1013,7 @@ DiskIo2ReadWriteDisk (
     FreePool (Task);
   }
 
+  gBS->RestoreTPL (OldTpl1); // MU_CHANGE TPL Symmetry
   gBS->RestoreTPL (OldTpl);
 
   return Status;