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The code in AcpiTableDxe handles the installation of FADT and FACS in both possible orders. In the [FADT, FACS] installation order, the FACS is at once linked into the FADT. In the [FACS, FADT] installation order, the FACS is stashed temporarily, and it is linked into the FADT when the FADT is installed later. According to the ACPI specification, *at most one* of FADT.FirmwareCtrl and FADT.XFirmwareCtrl may be nonzero. The code is aware of this requirement, and it never sets both of them to nonzero values at once. However, the code doesn't expect the following: - The caller first installs the FACS, which is stashed. The address that is saved happens to fall below 4GB. - The caller then installs a FADT, with a zero FirmwareCtrl field, and a nonzero (pre-populated) XFirmwareCtrl field. In this case the code sets FADT.FirmwareCtrl to the less-than-4GB address of the stashed FACS, and leaves the different nonzero value in FADT.XFirmwareCtrl. This violates the ACPI specification. Prevent this by always zeroing the field that we do *not* set. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Laszlo Ersek <lersek@redhat.com> Reviewed-by: Jiewen Yao <Jiewen.Yao@intel.com> git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16659 6f19259b-4bc3-4df7-8a09-765794883524