Re: [PATCHv4 15/18] x86/mm: Calculate direct mapping size

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Jun 26, 2018 at 05:22:42PM +0300, Kirill A. Shutemov wrote:
> The kernel needs to have a way to access encrypted memory. We have two
> option on how approach it:
> 
>  - Create temporary mappings every time kernel needs access to encrypted
>    memory. That's basically brings highmem and its overhead back.
> 
>  - Create multiple direct mappings, one per-KeyID. In this setup we
>    don't need to create temporary mappings on the fly -- encrypted
>    memory is permanently available in kernel address space.
> 
> We take the second approach as it has lower overhead.
> 
> It's worth noting that with per-KeyID direct mappings compromised kernel
> would give access to decrypted data right away without additional tricks
> to get memory mapped with the correct KeyID.
> 
> Per-KeyID mappings require a lot more virtual address space. On 4-level
> machine with 64 KeyIDs we max out 46-bit virtual address space dedicated
> for direct mapping with 1TiB of RAM. Given that we round up any
> calculation on direct mapping size to 1TiB, we effectively claim all
> 46-bit address space for direct mapping on such machine regardless of
> RAM size.
> 
> Increased usage of virtual address space has implications for KASLR:
> we have less space for randomization. With 64 TiB claimed for direct
> mapping with 4-level we left with 27 TiB of entropy to place
> page_offset_base, vmalloc_base and vmemmap_base.
> 
> 5-level paging provides much wider virtual address space and KASLR
> doesn't suffer significantly from per-KeyID direct mappings.
> 
> It's preferred to run MKTME with 5-level paging.


Why not make this a config dependency then?