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

[Thomas Gleixner]

On Wed, May 11 2022 at 08:49, Peter Zijlstra wrote:
> 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?

More bits are better :)

Seriously, I agree that restricting it to LAM57, which gives us 6 bits,
makes a lot of sense _and_ makes the whole thing way simpler.

So supporting both needs a truly good justification and a real world use
case.

Thanks,

        tglx