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Date: Fri, 14 Apr 2023 12:04:53 +0200
From: "Gerd Hoffmann" <kraxel@redhat.com>
To: devel@edk2.groups.io, zhiguang.liu@intel.com
Cc: "Dong, Eric" <eric.dong@intel.com>, "Ni, Ray" <ray.ni@intel.com>, 
	"Kumar, Rahul R" <rahul.r.kumar@intel.com>, "De, Debkumar" <debkumar.de@intel.com>, 
	"West, Catharine" <catharine.west@intel.com>
Subject: Re: [edk2-devel] [PATCH 2/2] UefiCpuPkg: Support 5 level page table in ResetVector
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> Using 5level paging when the below two conditions are both true:
>   1. CPU support 5level paging
>   2. Platform choose to use 5level paging. (by specifing macro USE_5_LEVEL_PAGE_TABLE)
> 
> There is an assumption that if platform choose to use 5level paging,
> then 5level paging should be supported by CPU. (Platform should know
> its CPU capability)

That assumption does not hold for virtual machines.  OVMF builds with
5-level paging support enabled should continue to work on CPUs without
5-level paging support.

> If Platform choose to use 4level paging at build time, we can save 4K space by not creating the 5 level page.

I'm fine with that.

But with 5-level paging enabled the reset vector should be able to
fallback to 4-level paging in case the CPU does not support 5-level
paging.

take care,
  Gerd