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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