Re: UefiCpuPkg CpuDxe GDT init question?

[Laszlo Ersek <lersek@redhat.com>]

Hi Andrew,

On 03/07/19 23:37, Andrew Fish via edk2-devel 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

I think you are right.

I'm missing the background on this too. I tried to see if any
justification was given in a git commit message, but according to "git
blame", this code dates back to the original addition of the driver,
namely commit a47463f28382 ("Add CPU DXE driver for IA32 & X64 processor
architectures.", 2009-05-27). The commit message is unhelpful (for 3119
lines added).

Thanks
Laszlo

> 
> 
> 
> 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
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