linux-kernel - Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

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Date:   Wed, 24 Aug 2022 17:41:49 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.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, linux-kselftest@...r.kernel.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>,
        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>, 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>,
        "Gupta, Pankaj" <pankaj.gupta@....com>
Subject: Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM
 guest private memory

On Tue, Aug 23, 2022 at 04:05:27PM +0000, Sean Christopherson wrote:
> On Tue, Aug 23, 2022, David Hildenbrand wrote:
> > On 19.08.22 05:38, Hugh Dickins wrote:
> > > On Fri, 19 Aug 2022, Sean Christopherson wrote:
> > >> On Thu, Aug 18, 2022, Kirill A . Shutemov wrote:
> > >>> On Wed, Aug 17, 2022 at 10:40:12PM -0700, Hugh Dickins wrote:
> > >>>> On Wed, 6 Jul 2022, Chao Peng wrote:
> > >>>> But since then, TDX in particular has forced an effort into preventing
> > >>>> (by flags, seals, notifiers) almost everything that makes it shmem/tmpfs.
> > >>>>
> > >>>> Are any of the shmem.c mods useful to existing users of shmem.c? No.
> > >>>> Is MFD_INACCESSIBLE useful or comprehensible to memfd_create() users? No.
> > >>
> > >> But QEMU and other VMMs are users of shmem and memfd.  The new features certainly
> > >> aren't useful for _all_ existing users, but I don't think it's fair to say that
> > >> they're not useful for _any_ existing users.
> > > 
> > > Okay, I stand corrected: there exist some users of memfd_create()
> > > who will also have use for "INACCESSIBLE" memory.
> > 
> > As raised in reply to the relevant patch, I'm not sure if we really have
> > to/want to expose MFD_INACCESSIBLE to user space. I feel like this is a
> > requirement of specific memfd_notifer (memfile_notifier) implementations
> > -- such as TDX that will convert the memory and MCE-kill the machine on
> > ordinary write access. We might be able to set/enforce this when
> > registering a notifier internally instead, and fail notifier
> > registration if a condition isn't met (e.g., existing mmap).
> >
> > So I'd be curious, which other users of shmem/memfd would benefit from
> > (MMU)-"INACCESSIBLE" memory obtained via memfd_create()?
> 
> I agree that there's no need to expose the inaccessible behavior via uAPI.  Making
> it a kernel-internal thing that's negotiated/resolved when KVM binds to the fd
> would align INACCESSIBLE with the UNMOVABLE and UNRECLAIMABLE flags (and any other
> flags that get added in the future).
> 
> AFAICT, the user-visible flag is a holdover from the early RFCs and doesn't provide
> any unique functionality.

That's also what I'm thinking. And I don't see problem immediately if
user has populated the fd at the binding time. Actually that looks an
advantage for previously discussed guest payload pre-loading.

> 
> If we go that route, we might want to have shmem/memfd require INACCESSIBLE to be
> set for the initial implementation.  I.e. disallow binding without INACCESSIBLE
> until there's a use case.

I can do that.

Chao