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From:   David Matlack <dmatlack@google.com>
Date:   Thu, 2 Nov 2023 09:28:23 -0700
Message-ID: <CALzav=eaVc5rzmHwnQr7aotyTKi9Agdte7NAL0NvBeE+f6zYoA@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 Thu, Nov 2, 2023 at 9:03=E2=80=AFAM Sean Christopherson <seanjc@google.c=
om> wrote:
>
> On Thu, Nov 02, 2023, Paolo Bonzini wrote:
> > On 10/31/23 23:39, David Matlack wrote:
> > > > > Maybe can you sketch out how you see this proposal being extensib=
le to
> > > > > using guest_memfd for shared mappings?
> > > > For in-place conversions, e.g. pKVM, no additional guest_memfd is n=
eeded.  What's
> > > > missing there is the ability to (safely) mmap() guest_memfd, e.g. K=
VM needs to
> > > > ensure there are no outstanding references when converting back to =
private.
> > > >
> > > > 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 gu=
est_memfd?
> >
> > For TDX/SNP, you could have a hook from KVM_SET_MEMORY_ATTRIBUTES to gu=
est
> > memory.  The hook would invalidate now-private parts if they have a VMA=
,
> > causing a SIGSEGV/EFAULT if the host touches them.
> >
> > It would forbid mappings from multiple gfns to a single offset of the
> > guest_memfd, because then the shared vs. private attribute would be tie=
d to
> > the offset.  This should not be a problem; for example, in the case of =
SNP,
> > the RMP already requires a single mapping from host physical address to
> > guest physical address.
>
> I don't see how this can work.  It's not a M:1 scenario (where M is multi=
ple gfns),
> it's a 1:N scenario (wheren N is multiple offsets).  The *gfn* doesn't ch=
ange on
> a conversion, what needs to change to do non-in-place conversion is the p=
fn, which
> is effectively the guest_memfd+offset pair.
>
> So yes, we *could* support non-in-place conversions within a single guest=
_memfd,
> but it would require a second offset,

Why can't KVM free the existing page at guest_memfd+offset and
allocate a new one when doing non-in-place conversions?

> at which point it makes sense to add a
> second file descriptor as well.  Userspace could still use a single guest=
_memfd
> instance, i.e. pass in the same file descriptor but different offsets.