Re: [edk2-devel] Access 64bit address space in 32bit mode

["Andrew Fish" <afish@apple.com>]

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Vincent,

Thanks! I’d forgotten about that path. 

The other answer is defer the work to a DXE driver that runs 64-bit x86.

Thanks,

Andrew Fish

> On Nov 9, 2022, at 10:58 AM, vincent zimmer <vincent.zimmer@gmail.com> wrote:
> 
> we have the challenge of 32-bit PEI needing to access 64-bit addresses to support 64-bit DXE/UEFI OS's in the capsule use-case scenario.  This is described in https://raw.githubusercontent.com/tianocore-docs/Docs/master/White_Papers/A_Tour_Beyond_BIOS_Capsule_Update_and_Recovery_in_EDK_II.pdf page 22 with code https://github.com/tianocore/edk2/tree/master/MdeModulePkg/Universal/CapsulePei
> 
> On Wed, Nov 9, 2022 at 9:21 AM Andrew Fish via groups.io <http://groups.io/> <afish=apple.com@groups.io <mailto:apple.com@groups.io>> wrote:
>> 
>>> On Nov 7, 2022, at 7:16 PM, Yoshinoya <yoshinoyatoko@163.com <mailto:yoshinoyatoko@163.com>> wrote:
>>> 
>>> Hello
>>> Is it possible to access 64bit address space in 32bit mode?
>>> 
>> 
>> I assume you are talking about x86?
>> 
>> 
>>> For example, opcode prefix 0x66/67 could let code running in 16bit mode to access 32bit data/address.
>>> 
>> 
>> This is more complex than just instruction prefix. You need the CPU to be setup in big real mode via the GDT, so you basically have a 32-bit environment setup via GDT etc. Also the prefix opcodes have different meaning in different modes. I don’t think there is a way to make 32-bit code access 64-bit data via instruction prefix even if a 64-bit GDT was setup with paging enabled.  
>> 
>>> Or, establishing page table is a must requirement for accessing 64bit address space.
>>> 
>> 
>> For x86 you have to have 64-bit versions of the IDT, GDT, and you need to enable paging to enter 64-bit Long Mode. 
>> 
>> In a 32-bit x86 world you can access up to 64 GB of physical memory via using 32-bit page table using PAE [1]. PAE is a 32-bit virtual address space, but with support for a 36-bit physical address. I think in the olden days of 32-bit x86 EFI servers would have custom EFI code that enabled paging in 32-bit and carved out a chunk of the 32-bit memory space that could be mapped to 36-bit physical addresses. I think this was platform specific code and I don’t know of any open source version. The 32-bit Long Mode EFI does not have paging enabled, so adding PAE means enabling paging yourself. 
>> 
>> The edk2 has the opposite version of this code so you can call 16-bit really mode (Legacy BIOS) from 32-bit Protected mode, or 64-bit Long Mode. This is the code to Thunk for 32-bit/64-bit mode to 16-bit code [2]/ 
>> 
>> [1] https://en.wikipedia.org/wiki/Physical_Address_Extensio <https://en.wikipedia.org/wiki/Physical_Address_Extension>n <https://en.wikipedia.org/wiki/Physical_Address_Extension>
>> [2] https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X86Thunk.c
>> <edk2.png>
>> edk2/Thunk16.nasm at master · tianocore/edk2
>> github.com
>>  <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X64/Thunk16.nasm>edk2/Thunk16.nasm at master · tianocore/edk2 <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X64/Thunk16.nasm>
>> github.com <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X64/Thunk16.nasm>
>> 
>> edk2/Thunk16.nasm at master · tianocore/edk2
>> github.com
>>  <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm>edk2/Thunk16.nasm at master · tianocore/edk2 <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm>
>> github.com <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm>
>> 
>> Thanks,
>> 
>> Andrew Fish
>> 
>> 
>>> Thanks
>>>  
>> 
>> 
>> 
> 
> 
> <edk2.png>


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