Date:   Tue, 15 Nov 2022 17:48:46 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Michael Roth <michael.roth@....com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Vlastimil Babka <vbabka@...e.cz>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>, tabba@...gle.com,
        mhocko@...e.com, Muchun Song <songmuchun@...edance.com>,
        wei.w.wang@...el.com
Subject: Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to
 create restricted user memory

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

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