Re: [edk2-devel] [PATCH v1 1/3] UefiCpuPkg/SecCore: Migrate page table to permanent memory

["Ni, Ray" <ray.ni@intel.com>]

> -----Original Message-----
> From: kraxel@redhat.com <kraxel@redhat.com>
> Sent: Wednesday, May 10, 2023 3:48 PM
> To: devel@edk2.groups.io; Ni, Ray <ray.ni@intel.com>
> Cc: Wu, Jiaxin <jiaxin.wu@intel.com>; Dong, Eric <eric.dong@intel.com>; Zeng,
> Star <star.zeng@intel.com>; Kumar, Rahul R <rahul.r.kumar@intel.com>
> Subject: Re: [edk2-devel] [PATCH v1 1/3] UefiCpuPkg/SecCore: Migrate page
> table to permanent memory
> 
> On Wed, May 10, 2023 at 02:48:52AM +0000, Ni, Ray wrote:
> > Gerd,
> > My understanding is that when code dereferences memory address, the code
> itself is responsible for
> > supplying the sign-extended linear address.
> > The page table creation logic still maps the entire linear memory space
> supported by the CPU.
> >
> > Why do you think covering the half of the space is better?
> 
> edk2 boot services operate on the assumption that everything is identity
> mapped, only runtime services know the concept of virtual addresses.
> 
> The lower half of the address space can be identity-mapped (virtual
> address == physical address).  The upper half can not, so I think it's
> better for efi boot services to restrict themself to the lower half.

Good point. I am convinced that 4-level paging only maps up to 2^47 address and
5-level only maps up to 2^56 address.

+@Kinney, Michael D, if he have other thoughts.

> 
> take care,
>   Gerd