Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

[Michael Roth]

On Thu, Nov 03, 2022 at 12:14:04AM +0300, Kirill A. Shutemov wrote:
> On Mon, Oct 31, 2022 at 12:47:38PM -0500, Michael Roth wrote:
> > 
> > In v8 there was some discussion about potentially passing the page/folio
> > and order as part of the invalidation callback, I ended up needing
> > something similar for SEV-SNP, and think it might make sense for other
> > platforms. This main reasoning is:
> > 
> >   1) restoring kernel directmap:
> > 
> >      Currently SNP (and I believe TDX) need to either split or remove kernel
> >      direct mappings for restricted PFNs, since there is no guarantee that
> >      other PFNs within a 2MB range won't be used for non-restricted
> >      (which will cause an RMP #PF in the case of SNP since the 2MB
> >      mapping overlaps with guest-owned pages)
> 
> That's news to me. Where the restriction for SNP comes from?

Sorry, missed your first question.

For SNP at least, the restriction is documented in APM Volume 2, Section
15.36.10, First row of Table 15-36 (preceeding paragraph has more
context). I forgot to mention this is only pertaining to writes by the
host to 2MB pages that contain guest-owned subpages, for reads it's
not an issue, but I think the implementation requirements end up being
the same either way:

  https://www.amd.com/system/files/TechDocs/24593.pdf

-Mike

> That's news to me. Where the restriction for SNP comes from? There's no
> such limitation on TDX side AFAIK?
> 
> Could you point me to relevant documentation if there's any?
> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov