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

[Chao Peng]

On Mon, Nov 14, 2022 at 04:16:32PM -0600, Michael Roth wrote:
> On Mon, Nov 14, 2022 at 06:28:43PM +0300, Kirill A. Shutemov wrote:
> > On Mon, Nov 14, 2022 at 03:02:37PM +0100, Vlastimil Babka wrote:
> > > On 11/1/22 16:19, Michael Roth wrote:
> > > > On Tue, Nov 01, 2022 at 07:37:29PM +0800, Chao Peng wrote:
> > > >> > 
> > > >> >   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)
> > > >> 
> > > >> Has the splitting and restoring been a well-discussed direction? I'm
> > > >> just curious whether there is other options to solve this issue.
> > > > 
> > > > For SNP it's been discussed for quite some time, and either splitting or
> > > > removing private entries from directmap are the well-discussed way I'm
> > > > aware of to avoid RMP violations due to some other kernel process using
> > > > a 2MB mapping to access shared memory if there are private pages that
> > > > happen to be within that range.
> > > > 
> > > > In both cases the issue of how to restore directmap as 2M becomes a
> > > > problem.
> > > > 
> > > > I was also under the impression TDX had similar requirements. If so,
> > > > do you know what the plan is for handling this for TDX?
> > > > 
> > > > There are also 2 potential alternatives I'm aware of, but these haven't
> > > > been discussed in much detail AFAIK:
> > > > 
> > > > a) Ensure confidential guests are backed by 2MB pages. shmem has a way 
> > > > to
> > > >    request 2MB THP pages, but I'm not sure how reliably we can guarantee
> > > >    that enough THPs are available, so if we went that route we'd 
> > > > probably
> > > >    be better off requiring the use of hugetlbfs as the backing store. 
> > > > But
> > > >    obviously that's a bit limiting and it would be nice to have the 
> > > > option
> > > >    of using normal pages as well. One nice thing with invalidation
> > > >    scheme proposed here is that this would "Just Work" if implement
> > > >    hugetlbfs support, so an admin that doesn't want any directmap
> > > >    splitting has this option available, otherwise it's done as a
> > > >    best-effort.
> > > > 
> > > > b) Implement general support for restoring directmap as 2M even when
> > > >    subpages might be in use by other kernel threads. This would be the
> > > >    most flexible approach since it requires no special handling during
> > > >    invalidations, but I think it's only possible if all the CPA
> > > >    attributes for the 2M range are the same at the time the mapping is
> > > >    restored/unsplit, so some potential locking issues there and still
> > > >    chance for splitting directmap over time.
> > > 
> > > I've been hoping that
> > > 
> > > c) using a mechanism such as [1] [2] where the goal is to group together
> > > these small allocations that need to increase directmap granularity so
> > > maximum number of large mappings are preserved.
> > 
> > As I mentioned in the other thread the restricted memfd can be backed by
> > secretmem instead of plain memfd. It already handles directmap with care.
> 
> It looks like it would handle direct unmapping/cleanup nicely, but it
> seems to lack fallocate(PUNCH_HOLE) support which we'd probably want to
> avoid additional memory requirements. I think once we added that we'd
> still end up needing some sort of handling for the invalidations.
> 
> Also, I know Chao has been considering hugetlbfs support, I assume by
> leveraging the support that already exists in shmem. Ideally SNP would
> be able to make use of that support as well, but relying on a separate
> backend seems likely to result in more complications getting there
> later.
> 
> > 
> > But I don't think it has to be part of initial restricted memfd
> > implementation. It is SEV-specific requirement and AMD folks can extend
> > implementation as needed later.
> 
> Admittedly the suggested changes to the invalidation mechanism made a
> lot more sense to me when I was under the impression that TDX would have
> similar requirements and we might end up with a common hook. Since that
> doesn't actually seem to be the case, it makes sense to try to do it as
> a platform-specific hook for SNP.
> 
> I think, given a memslot, a GFN range, and kvm_restricted_mem_get_pfn(),
> we should be able to get the same information needed to figure out whether
> the range is backed by huge pages or not. I'll see how that works out
> instead.

Sounds a viable solution, just that kvm_restricted_mem_get_pfn() will
only give you the ability to check a page, not a range. But you can
still call it many times I think.

The invalidation callback will be still needed, it gives you the chance
to do the restoring.

Chao
> 
> Thanks,
> 
> Mike
> 
> > 
> > -- 
> >   Kiryl Shutsemau / Kirill A. Shutemov