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. The specification also
restricts usage of TPL_HIGH_LEVEL to the firmware itself.
However, nothing actually prevents a UEFI application from calling
gBS->RaiseTPL(TPL_HIGH_LEVEL) and then violating the invariant by
enabling interrupts via the STI or equivalent instruction. Some
versions of the Microsoft Windows bootloader are known to do this.
NestedInterruptTplLib maintains the invariant that interrupts are
disabled at TPL_HIGH_LEVEL (even when performing the dark art of
deliberately manipulating the stack so that IRET will return with
interrupts still disabled), but does not itself rely on external code
maintaining this invariant.
Relax the assertion that the interrupted TPL is below TPL_HIGH_LEVEL
to an error message, to allow UEFI applications such as these versions
of the Microsoft Windows bootloader to continue to function.
Debugged-by: Gerd Hoffmann <kraxel@redhat.com>
Debugged-by: Laszlo Ersek <lersek@redhat.com>
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=2189136
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
NestedInterruptTplLib relies on CPU interrupts being disabled to
guarantee exclusive (and hence atomic) access to the shared state in
IsrState. Nothing in the calling interrupt handler should have
re-enabled interrupts before calling NestedInterruptRestoreTPL(), and
the loop in NestedInterruptRestoreTPL() itself maintains the invariant
that interrupts are disabled at the start of each iteration.
Add assertions to clarify this invariant, and expand the comments
around the calls to RestoreTPL() and DisableInterrupts() to clarify
the expectations around enabling and disabling interrupts.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
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 underrun, while continuing to allow nested
interrupts to occur.
This can be achieved by allowing, when provably safe to do so, an
inner interrupt handler to return from the interrupt without restoring
the TPL and with interrupts remaining disabled after IRET, with the
deferred call to RestoreTPL() then being issued from the outer
interrupt handler. This is necessarily messy and involves direct
manipulation of the interrupt stack frame, and so should not be
implemented as open-coded logic within each interrupt handler.
Add the Nested Interrupt TPL Library (NestedInterruptTplLib) to
provide helper functions that can be used by nested interrupt handlers
in place of RaiseTPL()/RestoreTPL().
Example call tree for a timer interrupt occurring at TPL_APPLICATION
with a nested timer interrupt that makes its own call to RestoreTPL():
outer TimerInterruptHandler()
InterruptedTPL == TPL_APPLICATION
...
IsrState->InProgressRestoreTPL = TPL_APPLICATION;
gBS->RestoreTPL (TPL_APPLICATION);
EnableInterrupts();
dispatch a TPL_CALLBACK event
gEfiCurrentTpl = TPL_CALLBACK;
nested timer interrupt occurs
inner TimerInterruptHandler()
InterruptedTPL == TPL_CALLBACK
...
IsrState->InProgressRestoreTPL = TPL_CALLBACK;
gBS->RestoreTPL (TPL_CALLBACK);
EnableInterrupts();
DisableInterrupts();
IsrState->InProgressRestoreTPL = TPL_APPLICATION;
IRET re-enables interrupts
... finish dispatching TPL_CALLBACK events ...
gEfiCurrentTpl = TPL_APPLICATION;
DisableInterrupts();
IsrState->InProgressRestoreTPL = 0;
sees IsrState->DeferredRestoreTPL == FALSE and returns
IRET re-enables interrupts
Example call tree for a timer interrupt occurring at TPL_APPLICATION
with a nested timer interrupt that defers its call to RestoreTPL() to
the outer instance of the interrupt handler:
outer TimerInterruptHandler()
InterruptedTPL == TPL_APPLICATION
...
IsrState->InProgressRestoreTPL = TPL_APPLICATION;
gBS->RestoreTPL (TPL_APPLICATION);
EnableInterrupts();
dispatch a TPL_CALLBACK event
... finish dispatching TPL_CALLBACK events ...
gEfiCurrentTpl = TPL_APPLICATION;
nested timer interrupt occurs
inner TimerInterruptHandler()
InterruptedTPL == TPL_APPLICATION;
...
sees InterruptedTPL == IsrState->InProgressRestoreTPL
IsrState->DeferredRestoreTPL = TRUE;
DisableInterruptsOnIret();
IRET returns without re-enabling interrupts
DisableInterrupts();
IsrState->InProgressRestoreTPL = 0;
sees IsrState->DeferredRestoreTPL == TRUE and loops
IsrState->InProgressRestoreTPL = TPL_APPLICATION;
gBS->RestoreTPL (TPL_APPLICATION); <-- deferred call
EnableInterrupts();
DisableInterrupts();
IsrState->InProgressRestoreTPL = 0;
sees IsrState->DeferredRestoreTPL == FALSE and returns
IRET re-enables interrupts
Cc: Paolo Bonzini <pbonzini@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=4162
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Michael Brown