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

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Message-ID: <20221102212656.6giugw542jdxsvhh@amd.com>
Date:   Wed, 2 Nov 2022 16:26:56 -0500
From:   Michael Roth <michael.roth@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     Chao Peng <chao.p.peng@...ux.intel.com>, <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>,
        Vlastimil Babka <vbabka@...e.cz>,
        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 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@google.com/

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