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

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Date:   Sat, 20 Aug 2022 22:15:32 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
cc:     Hugh Dickins <hughd@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Chao Peng <chao.p.peng@...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>,
        Sean Christopherson <seanjc@...gle.com>,
        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,
        david@...hat.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 Sat, 20 Aug 2022, Kirill A. Shutemov wrote:
> 
> Yes, INACCESSIBLE is increase of complexity which you do not want to deal
> with in shmem.c. It get it.

It's not so much that INACCESSIBLE increases the complexity of
memfd/shmem/tmpfs, as that it is completely foreign to it.

And by handling all those foreign needs at the KVM end (where you
can be sure that the mem attached to the fd is INACCESSIBLE because
you have given nobody access to it - no handshaking with 3rd party
required).

> 
> I will try next week to rework it as shim to top of shmem. Does it work
> for you?

Yes, please do, thanks.  It's a compromise between us: the initial TDX
case has no justification to use shmem at all, but doing it that way
will help you with some of the infrastructure, and will probably be
easiest for KVM to extend to other more relaxed fd cases later.

> 
> But I think it is wrong to throw it over the fence to KVM folks and say it
> is your problem. Core MM has to manage it.

We disagree on who is throwing over the fence to whom :)

Core MM should manage the core MM parts and KVM should manage the KVM
parts.  What makes this rather different from most driver usage of MM,
is that KVM seems likely to use a great proportion of memory this way.
With great memory usage comes great responsibility: I don't think
all those flags and seals and notifiers let KVM escape from that.

Hugh