Compatibility Consideration

1. OS compatibility

   If the heap guard is enabled (especially pool guard), the UEFI memory map is fragmented. We notice that the fragmentation may cause OS boot or installation fail. For example, Windows Boot Manager sets the maximum number of global memory descriptor for a 64-bit UEFI system at 512. (https://support.microsoft.com/en-us/help/4020050/blinitializelibrary-failed-xxx-error-when-you-install-or-start-an-oper)

   For NULL pointer detection, BIT7 of PcdNullPointerDetectionPropertyMask(disables NULL pointer detection just after EndOfDxe) is a workaround for those unsolvable NULL access issues in Option ROM, OS boot loader, such as Windows 7* boot on Qemu*.

2. 3rd part driver (PCI Option ROM)

   When the heap guard is enabled, we observed the #PF (Page Fault) exception happens and the instruction address is within a 3rd part PCI Option ROM.

3. Legacy Compatible Support Module (CSM)

   The legacy CSM module may need access the zero address and the first 4KiB memory, such as Interrupt Vector Table(IVT) and BIOS Data Area (BDA). Whenever the legacy module accesses the first 4KiB memory, the code must be included by ACCESS_PAGE0_CODE() macro. (https://github.com/tianocore/edk2/blob/master/IntelFrameworkPkg/Include/Protocol/LegacyBios.h). For example, when the LegacyBios driver accesses IVT region (https://github.com/tianocore/edk2/blob/master/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/Thunk.c) or when the LegacyKeyboard driver accesses BDA region (https://github.com/tianocore/edk2/blob/master/IntelFrameworkModulePkg/Csm/BiosThunk/KeyboardDxe/BiosKeyboard.c).

   The platform specific LegacyBiosPlatform driver also need update to add ACCESS_PAGE0_CODE() macro around the code to update IVT or BDA.

4. CPU driver

   The EDK II DxeCore relies on CPU driver EFI_CPU_ARCH_PROTOCOL.SetMemoryAttributes(https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Protocol/Cpu.h) to update the page table (https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/CpuDxe/CpuPageTable.c). The EDK II PiSmmCore also relies on PiSmmCpu driver EDKII_SMM_MEMORY_ATTRIBUTE_PROTOCOL(https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Include/Protocol/SmmMemoryAttribute.h)to update the page table (https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c). If a platform uses its own CPU driver instead of the open source one, this platform need add the page table update capability in the CPU driver.