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Date:   Thu, 3 Nov 2022 19:30:12 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Michael Roth <michael.roth@amd.com>
Cc:     Chao Peng <chao.p.peng@linux.intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
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        Sean Christopherson <seanjc@google.com>,
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        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Hugh Dickins <hughd@google.com>,
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        Vlastimil Babka <vbabka@suse.cz>,
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        Yu Zhang <yu.c.zhang@linux.intel.com>,
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Subject: Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to
 create restricted user memory
Message-ID: <20221103163012.25zof5lyxvizltwl@box.shutemov.name>
References: <20221025151344.3784230-1-chao.p.peng@linux.intel.com>
 <20221025151344.3784230-2-chao.p.peng@linux.intel.com>
 <20221031174738.fklhlia5fmaiinpe@amd.com>
 <20221102211404.l5whyif3j3k67fv2@box.shutemov.name>
 <20221102220700.5u4mj7fm37m6ust2@amd.com>
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On Wed, Nov 02, 2022 at 05:07:00PM -0500, Michael Roth wrote:
> 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

Looks like you wanted restricted memfd to be backed by secretmem rather
then normal memfd. It would help preserve directmap.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov