linux-kernel - Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

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Date:   Fri, 3 Sep 2021 12:15:51 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Andi Kleen <ak@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
 memory

On 9/3/21 12:14 PM, Kirill A. Shutemov wrote:
> On Thu, Sep 02, 2021 at 08:33:31PM +0000, Sean Christopherson wrote:
>> Would requiring the size to be '0' at F_SEAL_GUEST time solve that problem?
> 
> I guess. Maybe we would need a WRITE_ONCE() on set. I donno. I will look
> closer into locking next.

We can decisively eliminate this sort of failure by making the switch
happen at open time instead of after.  For a memfd-like API, this would
be straightforward.  For a filesystem, it would take a bit more thought.