[PATCH v2 0/2] OvmfPkg: Relax assertion that interrupts do not occur at 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. 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.

Ref: https://bugzilla.redhat.com/show_bug.cgi?id=2189136
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Oliver Steffen <osteffen@redhat.com>
Cc: Pawel Polawski <ppolawsk@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>

Michael Brown (2):
  OvmfPkg: Clarify invariants for NestedInterruptTplLib
  OvmfPkg: Relax assertion that interrupts do not occur at
    TPL_HIGH_LEVEL

 OvmfPkg/Library/NestedInterruptTplLib/Tpl.c | 31 +++++++++++++++++----
 1 file changed, 26 insertions(+), 5 deletions(-)

-- 
2.39.0