Re: [PATCH 2/2] UefiCpuPkg/CpuDxe: Guarantee GDT is below 4GB

["Laszlo Ersek" <lersek@redhat.com>]

On 03/16/21 04:33, Ray Ni wrote:
> REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3233
> 
> GDT needs to be allocated below 4GB in 64bit environment
> because AP needs it for entering to protected mode.
> CPU running in big real mode cannot access above 4GB GDT.
> 
> But CpuDxe driver contains below code:
>   gdt = AllocateRuntimePool (sizeof (GdtTemplate) + 8);
>   .....
>   gdtPtr.Base = (UINT32)(UINTN)(VOID*) gdt;
> 
> The AllocateRuntimePool() may allocate memory above 4GB.
> Thus, we cannot use AllocateRuntimePool (), instead,
> we should use AllocatePages() to make sure GDT is below 4GB space.
> 
> Signed-off-by: Ray Ni <ray.ni@intel.com>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Rahul Kumar <rahul1.kumar@intel.com>
> ---
>  UefiCpuPkg/CpuDxe/CpuGdt.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/UefiCpuPkg/CpuDxe/CpuGdt.c b/UefiCpuPkg/CpuDxe/CpuGdt.c
> index 322ce87142..98d5551702 100644
> --- a/UefiCpuPkg/CpuDxe/CpuGdt.c
> +++ b/UefiCpuPkg/CpuDxe/CpuGdt.c
> @@ -124,15 +124,26 @@ InitGlobalDescriptorTable (
>    VOID
>    )
>  {
> +  EFI_STATUS            Status;
>    GDT_ENTRIES           *Gdt;
>    IA32_DESCRIPTOR       Gdtr;
> +  EFI_PHYSICAL_ADDRESS  Memory;
>  
>    //
> -  // Allocate Runtime Data for the GDT
> -  //
> -  Gdt = AllocateRuntimePool (sizeof (gGdtTemplate) + 8);
> -  ASSERT (Gdt != NULL);
> -  Gdt = ALIGN_POINTER (Gdt, 8);
> +  // Allocate Runtime Data below 4GB for the GDT
> +  // AP uses the same GDT when it's waken up from real mode so

(1) s/waken/woken/

> +  // the GDT needs to be below 4GB.
> +  //
> +  Memory = SIZE_4GB - 1;
> +  Status = gBS->AllocatePages (
> +                  AllocateMaxAddress,
> +                  EfiRuntimeServicesData,
> +                  EFI_SIZE_TO_PAGES (sizeof (gGdtTemplate)),
> +                  &Memory
> +                  );
> +  ASSERT_EFI_ERROR (Status);
> +  ASSERT ((Memory != 0) && (Memory < SIZE_4GB));

(2) Can we drop the (Memory < SIZE_4GB) sub-condition? That should be
guaranteed by the UEFI spec.

> +  Gdt = (GDT_ENTRIES *) (UINTN) Memory;
>  
>    //
>    // Initialize all GDT entries
> 

Anyway...

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo