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

Kirill A. Shutemov
2022-05-12T17:01:15Z

On Thu, May 12, 2022 at 05:42:58PM +0200, Thomas Gleixner wrote:
> 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.

I asked the question before[1]. Basically, more bits more better:

For HWASAN #bits == detection probability.
For MarkUS #bits == exponential cost reduction

I would really like to have only LAM_U57, but IIUC 6 bits is not always
enough.

Dmitry, could you elaborate?

[1] https://mobile.twitter.com/dvyukov/status/1342019823400837120
--
Kirill A. Shutemov