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? There's no
> such limitation on TDX side AFAIK?
> 
> Could you point me to relevant documentation if there's any?

I could be mistaken, I haven't looked into the specific documentation and was
going off of this discussion from a ways back:

  https://lore.kernel.org/all/YWb8WG6Ravbs1nbx@xxxxxxxxxx/

Sean, is my read of that correct? Do you happen to know where there's
some documentation on that for the TDX side?

Thanks,

Mike

> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov