linux-kernel - Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

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Date:   Mon, 17 Oct 2022 22:58:56 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Fuad Tabba <tabba@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...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>,
        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, aarcange@...hat.com, ddutile@...hat.com,
        dhildenb@...hat.com, Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>, wei.w.wang@...el.com,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

On Mon, Oct 17, 2022 at 11:31:19AM +0100, Fuad Tabba wrote:
> Hi,
> 
> > >
> > > Actually, for pKVM, there is no need for the guest memory to be
> > > GUP'able at all if we use the new inaccessible_get_pfn().
> >
> > If pKVM can use inaccessible_get_pfn() to get pfn and can avoid GUP (I
> > think that is the major concern?), do you see any other gap from
> > existing API?
> 
> Actually for this part no, there aren't any gaps and
> inaccessible_get_pfn() is sufficient.

Thanks for the confirmation.

> 
> > > This of
> > > course goes back to what I'd mentioned before in v7; it seems that
> > > representing the memslot memory as a file descriptor should be
> > > orthogonal to whether the memory is shared or private, rather than a
> > > private_fd for private memory and the userspace_addr for shared
> > > memory. The host can then map or unmap the shared/private memory using
> > > the fd, which allows it more freedom in even choosing to unmap shared
> > > memory when not needed, for example.
> >
> > Using both private_fd and userspace_addr is only needed in TDX and other
> > confidential computing scenarios, pKVM may only use private_fd if the fd
> > can also be mmaped as a whole to userspace as Sean suggested.
> 
> That does work in practice, for now at least, and is what I do in my
> current port. However, the naming and how the API is defined as
> implied by the name and the documentation. By calling the field
> private_fd, it does imply that it should not be mapped, which is also
> what api.rst says in PATCH v8 5/8. My worry is that in that case pKVM
> would be mis/ab-using this interface, and that future changes could
> cause unforeseen issues for pKVM.

That is fairly enough. We can change the naming and the documents.

> 
> Maybe renaming this to something like "guest_fp", and specifying in
> the documentation that it can be restricted, e.g., instead of "the
> content of the private memory is invisible to userspace" something
> along the lines of  "the content of the guest memory may be restricted
> to userspace".

Some other candidates in my mind:
- restricted_fd: to pair with the mm side restricted_memfd
- protected_fd: as Sean suggested before
- fd: how it's explained relies on the memslot.flag.

Thanks,
Chao
> 
> What do you think?
> 
> Cheers,
> /fuad
> 
> >
> > Thanks,
> > Chao
> > >
> > > Cheers,
> > > /fuad