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Date:   Tue, 13 Sep 2022 09:44:27 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>,
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        "J . Bruce Fields" <bfields@fieldses.org>,
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        Muchun Song <songmuchun@bytedance.com>,
        "Gupta, Pankaj" <pankaj.gupta@amd.com>,
        Elena Reshetova <elena.reshetova@intel.com>
Subject: Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM
 guest private memory
Message-ID: <YyBQ+wzPtGwwRB/U@google.com>
References: <20220706082016.2603916-1-chao.p.peng@linux.intel.com>
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 <20220820002700.6yflrxklmpsavdzi@box.shutemov.name>
 <c194262b-b634-4baf-abf0-dc727e8f1d7@google.com>
 <20220831142439.65q2gi4g2d2z4ofh@box.shutemov.name>
 <20220908011037.ez2cdorthqxkerwk@box.shutemov.name>
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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.