Re: UefiCpuPkg CpuDxe GDT init question?

[Andrew Fish <afish@apple.com>]

Actually it looks like the the CpuDxe driver is coded to only run if it it is loaded under 4 GB? Is that following the spec? Is that intentional?

I noticed that SetCodeSelector is coded to use a far jump and that is a 32-bit absolute value? Note [rsp+4]
https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/CpuDxe/X64/CpuAsm.nasm#L28
ASM_PFX(SetCodeSelector):
    sub     rsp, 0x10
    lea     rax, [setCodeSelectorLongJump]
    mov     [rsp], rax
    mov     [rsp+4], cx
    jmp     dword far [rsp]
setCodeSelectorLongJump:
    add     rsp, 0x10
    ret


Thanks,

Andrew Fish

> On Mar 7, 2019, at 2:37 PM, Andrew Fish <afish@apple.com> wrote:
> 
> I'm trying to understand why gdtPtr.Base is casting to (UINT32)? 
> 1) gdtPtr.Base is a a UINTN
> 2) It is legal for AllocateRuntimePool() to return an address > 4GB
> 
> It seems like the code should just cast to (UINTN)?
> 
> 
> https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/CpuDxe/CpuGdt.c#L151 <https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/CpuDxe/CpuGdt.c#L151>
> 
> 
> 
> VOID
> InitGlobalDescriptorTable (
>   VOID
>   )
> {
>   GDT_ENTRIES *gdt;
>   IA32_DESCRIPTOR gdtPtr;
> 
>   //
>   // Allocate Runtime Data for the GDT
>   //
>   gdt = AllocateRuntimePool (sizeof (GdtTemplate) + 8);
>   ASSERT (gdt != NULL);
>   gdt = ALIGN_POINTER (gdt, 8);
> 
>   //
>   // Initialize all GDT entries
>   //
>   CopyMem (gdt, &GdtTemplate, sizeof (GdtTemplate));
> 
>   //
>   // Write GDT register
>   //
>   gdtPtr.Base = (UINT32)(UINTN)(VOID*) gdt;
>   gdtPtr.Limit = (UINT16) (sizeof (GdtTemplate) - 1);
>   AsmWriteGdtr (&gdtPtr);
> 
> Thanks,
> 
> Andrew Fish