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

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Message-ID: <YyBQ+wzPtGwwRB/U@google.com>
Date:   Tue, 13 Sep 2022 09:44:27 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.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>,
        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>,
        Elena Reshetova <elena.reshetova@...el.com>
Subject: Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM
 guest private memory

On Thu, Sep 08, 2022, Kirill A. Shutemov wrote:
> On Wed, Aug 31, 2022 at 05:24:39PM +0300, Kirill A . Shutemov wrote:
> > On Sat, Aug 20, 2022 at 10:15:32PM -0700, Hugh Dickins wrote:
> > > > 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.
> > 
> > Okay, below is my take on the shim approach.
> > 
> > I don't hate how it turned out. It is easier to understand without
> > callback exchange thing.
> > 
> > The only caveat is I had to introduce external lock to protect against
> > race between lookup and truncate.

As before, I think this lock is unnecessary.  Or at least it's unnessary to hold
the lock across get/put.  The ->invalidate() call will ensure that the pfn is
never actually used if get() races with truncation.

Switching topics, what actually prevents mmapp() on the shim?  I tried to follow,
but I don't know these areas well enough.