Re: [RFCv2 00/10] Linear Address Masking enabling

[Peter Zijlstra]

On Wed, May 11, 2022 at 05:27:40AM +0300, Kirill A. Shutemov wrote:
> Hi all. Here's long overdue update on LAM enabling.
> 
> # Description #
> 
> Linear Address Masking[1] (LAM) modifies the checking that is applied to
> 64-bit linear addresses, allowing software to use of the untranslated
> address bits for metadata.
> 
> The patchset brings support for LAM for userspace addresses.
> 
> The most sensitive part of enabling is change in tlb.c, where CR3 flags
> get set. Please take a look that what I'm doing makes sense.
> 
> The feature competes for bits with 5-level paging: LAM_U48 makes it
> impossible to map anything about 47-bits. The patchset made these
> capability mutually exclusive: whatever used first wins. LAM_U57 can be
> combined with mappings above 47-bits.

So aren't we creating a problem with LAM_U48 where programs relying on
it are of limited sustainability?

Any such program simply *cannot* run on 5 level pagetables. Why do we
want to do this?