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From:   David Matlack <dmatlack@google.com>
Date:   Tue, 31 Oct 2023 15:39:42 -0700
Message-ID: <CALzav=d9eXZfK=op7A=UftbpuPpUbxqV6CmkqqxxBNuNsUU4nw@mail.gmail.com>
Subject: Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Xiaoyao Li <xiaoyao.li@intel.com>,
        Xu Yilun <yilun.xu@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        Fuad Tabba <tabba@google.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Anish Moorthy <amoorthy@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Isaku Yamahata <isaku.yamahata@intel.com>,
        =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= <mic@digikod.net>,
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        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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On Tue, Oct 31, 2023 at 2:36=E2=80=AFPM Sean Christopherson <seanjc@google.=
com> wrote:
> On Tue, Oct 31, 2023, David Matlack wrote:
> > On 2023-10-27 11:21 AM, Sean Christopherson wrote:
> > > Introduce an ioctl(), KVM_CREATE_GUEST_MEMFD, to allow creating file-=
based
> > > memory that is tied to a specific KVM virtual machine and whose prima=
ry
> > > purpose is to serve guest memory.
>
> > Maybe can you sketch out how you see this proposal being extensible to
> > using guest_memfd for shared mappings?
>
> For in-place conversions, e.g. pKVM, no additional guest_memfd is needed.=
  What's
> missing there is the ability to (safely) mmap() guest_memfd, e.g. KVM nee=
ds to
> ensure there are no outstanding references when converting back to privat=
e.
>
> For TDX/SNP, assuming we don't find a performant and robust way to do in-=
place
> conversions, a second fd+offset pair would be needed.

Is there a way to support non-in-place conversions within a single guest_me=
mfd?