Re: [PATCH 1/3] OvmfPkg/PlatformInitLib: check PcdUse1GPageTable

["Ard Biesheuvel" <ardb@kernel.org>]

On Wed, 17 May 2023 at 12:24, Gerd Hoffmann <kraxel@redhat.com> wrote:
>
> If PcdUse1GPageTable is not enabled restrict the physical address space
> used to 1TB, to limit the amount of memory needed for identity mapping
> page tables.
>
> The same already happens in case the processor has no support for
> gigabyte pages.
>

Apologies for the noob question, but does this mean EDK2 maps the
entire address space 1:1, not just the region that as any DRAM in it?


> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf | 1 +
>  OvmfPkg/Library/PlatformInitLib/MemDetect.c         | 5 +++++
>  2 files changed, 6 insertions(+)
>
> diff --git a/OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf b/OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf
> index 86a82ad3e084..5a79d95b689c 100644
> --- a/OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf
> +++ b/OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf
> @@ -58,6 +58,7 @@ [LibraryClasses.X64]
>
>  [Pcd]
>    gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress
> +  gEfiMdeModulePkgTokenSpaceGuid.PcdUse1GPageTable
>
>  [FixedPcd]
>    gUefiOvmfPkgTokenSpaceGuid.PcdOvmfWorkAreaBase
> diff --git a/OvmfPkg/Library/PlatformInitLib/MemDetect.c b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> index acf90b4e93fd..1102b00ecbf0 100644
> --- a/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> +++ b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> @@ -663,6 +663,11 @@ PlatformAddressWidthFromCpuid (
>        PhysBits = 40;
>      }
>
> +    if (!FixedPcdGetBool (PcdUse1GPageTable) && (PhysBits > 40)) {
> +      DEBUG ((DEBUG_INFO, "%a: limit PhysBits to 40 (PcdUse1GPageTable is false)\n", __func__));
> +      PhysBits = 40;
> +    }
> +
>      PlatformInfoHob->PhysMemAddressWidth = PhysBits;
>      PlatformInfoHob->FirstNonAddress     = LShiftU64 (1, PlatformInfoHob->PhysMemAddressWidth);
>    }
> --
> 2.40.1
>