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/** @file
Handle raising and lowering TPL from within nested interrupt handlers.
Allows interrupt handlers to safely raise and lower the TPL to
dispatch event notifications, correctly allowing for nested
interrupts to occur without risking stack exhaustion.
Copyright (C) 2022, Fen Systems Ltd.
SPDX-License-Identifier: BSD-2-Clause-Patent
**/
#pragma once
#include <Uefi/UefiBaseType.h>
#include <Uefi/UefiSpec.h>
#include <Protocol/DebugSupport.h>
///
/// State shared between all invocations of a nested interrupt handler.
///
typedef struct {
///
/// Highest TPL that is currently the target of a call to
/// RestoreTPL() by an instance of this interrupt handler.
///
EFI_TPL InProgressRestoreTPL;
///
/// Flag used to defer a call to RestoreTPL() from an inner instance
/// of the interrupt handler to an outer instance of the same
/// interrupt handler.
///
BOOLEAN DeferredRestoreTPL;
} NESTED_INTERRUPT_STATE;
/**
Raise the task priority level to TPL_HIGH_LEVEL.
@param None.
@return The task priority level at which the interrupt occurred.
**/
EFI_TPL
EFIAPI
NestedInterruptRaiseTPL (
VOID
);
/**
Lower the task priority back to the value at which the interrupt
occurred.
This is unfortunately messy. UEFI requires us to support nested
interrupts, but provides no way for an interrupt handler to call
RestoreTPL() without implicitly re-enabling interrupts. In a
virtual machine, it is possible for a large burst of interrupts to
arrive. We must prevent such a burst from leading to stack
exhaustion, while continuing to allow nested interrupts to occur.
Since nested interrupts are permitted, an interrupt handler may be
invoked as an inner interrupt handler while an outer instance of the
same interrupt handler is still inside its call to RestoreTPL().
To avoid stack exhaustion, this call may therefore (when provably
safe to do so) defer the actual TPL lowering to be performed by an
outer instance of the same interrupt handler.
@param InterruptedTPL The task priority level at which the interrupt
occurred, as previously returned from
NestedInterruptRaiseTPL().
@param SystemContext A pointer to the system context when the
interrupt occurred.
@param IsrState A pointer to the state shared between all
invocations of the nested interrupt handler.
**/
VOID
EFIAPI
NestedInterruptRestoreTPL (
IN EFI_TPL InterruptedTPL,
IN OUT EFI_SYSTEM_CONTEXT SystemContext,
IN OUT NESTED_INTERRUPT_STATE *IsrState
);
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