linux-kernel - Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory

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Message-Id: <4ae11ddc-0fe6-4644-a30e-006d99dba456@www.fastmail.com>
Date:   Wed, 06 Apr 2022 11:42:17 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Sean Christopherson" <seanjc@...gle.com>
Cc:     "Quentin Perret" <qperret@...gle.com>,
        "Steven Price" <steven.price@....com>,
        "Chao Peng" <chao.p.peng@...ux.intel.com>,
        "kvm list" <kvm@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        "Linux API" <linux-api@...r.kernel.org>, qemu-devel@...gnu.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>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Hugh Dickins" <hughd@...gle.com>,
        "Jeff Layton" <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Mike Rapoport" <rppt@...nel.org>,
        "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>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Andi Kleen" <ak@...ux.intel.com>,
        "David Hildenbrand" <david@...hat.com>,
        "Marc Zyngier" <maz@...nel.org>, "Will Deacon" <will@...nel.org>
Subject: Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest
 private memory



On Tue, Apr 5, 2022, at 11:30 AM, Sean Christopherson wrote:
> On Tue, Apr 05, 2022, Andy Lutomirski wrote:

>
>> resume guest
>> *** host -> hypervisor -> guest ***
>> Guest unshares the page.
>> *** guest -> hypervisor ***
>> Hypervisor removes PTE.  TLBI.
>> *** hypervisor -> guest ***
>> 
>> Obviously considerable cleverness is needed to make a virt IOMMU like this
>> work well, but still.
>> 
>> Anyway, my suggestion is that the fd backing proposal get slightly modified
>> to get it ready for multiple subtypes of backing object, which should be a
>> pretty minimal change.  Then, if someone actually needs any of this
>> cleverness, it can be added later.  In the mean time, the
>> pread()/pwrite()/splice() scheme is pretty good.
>
> Tangentially related to getting private-fd ready for multiple things, 
> what about
> implementing the pread()/pwrite()/splice() scheme in pKVM itself?  I.e. 
> read() on
> the VM fd, with the offset corresponding to gfn in some way.
>

Hmm, could make sense.