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

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Message-ID: <20220713075738.GC2831541@chaop.bj.intel.com>
Date:   Wed, 13 Jul 2022 15:57:38 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     "Gupta, Pankaj" <pankaj.gupta@....com>
Cc:     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,
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        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
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        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>,
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        Vishal Annapurve <vannapurve@...gle.com>,
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        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM
 guest private memory

On Wed, Jul 13, 2022 at 05:58:32AM +0200, Gupta, Pankaj wrote:
> 
> > This is the v7 of this series which tries to implement the fd-based KVM
> > guest private memory. The patches are based on latest kvm/queue branch
> > commit:
> > 
> >    b9b71f43683a (kvm/queue) KVM: x86/mmu: Buffer nested MMU
> > split_desc_cache only by default capacity
> > 
> > Introduction
> > ------------
> > In general this patch series introduce fd-based memslot which provides
> > guest memory through memory file descriptor fd[offset,size] instead of
> > hva/size. The fd can be created from a supported memory filesystem
> > like tmpfs/hugetlbfs etc. which we refer as memory backing store. KVM
> 
> Thinking a bit, As host side fd on tmpfs or shmem will store memory on host
> page cache instead of mapping pages into userspace address space. Can we hit
> double (un-coordinated) page cache problem with this when guest page cache
> is also used?

This is my understanding: in host it will be indeed in page cache (in
current shmem implementation) but that's just the way it allocates and
provides the physical memory for the guest. In guest, guest OS will not
see this fd (absolutely), it only sees guest memory, on top of which it
can build its own page cache system for its own file-mapped content but
that is unrelated to host page cache.

Chao
> 
> Thanks,
> Pankaj
>