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Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
 memory
To:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Borislav Petkov <bp@alien8.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>,
        Andi Kleen <ak@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Varad Gautam <varad.gautam@suse.com>,
        Dario Faggioli <dfaggioli@suse.com>, x86@kernel.org,
        linux-mm@kvack.org, linux-coco@lists.linux.dev,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>
References: <20210824005248.200037-1-seanjc@google.com>
 <20210902184711.7v65p5lwhpr2pvk7@box.shutemov.name>
 <YTE1GzPimvUB1FOF@google.com>
 <20210903191414.g7tfzsbzc7tpkx37@box.shutemov.name>
From:   Andy Lutomirski <luto@kernel.org>
Message-ID: <02806f62-8820-d5f9-779c-15c0e9cd0e85@kernel.org>
Date:   Fri, 3 Sep 2021 12:15:51 -0700
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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.