linux-kernel - Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

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Date:   Thu, 2 Sep 2021 11:41:07 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Andi Kleen <ak@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
 memory

>>
>> In principle, you could actually initialize a TDX guest with all of its
>> memory shared and all of it mapped in the host IOMMU.  When a guest
>> turns some pages private, user code could punch a hole in the memslot,
>> allocate private memory at that address, but leave the shared backing
>> store in place and still mapped in the host IOMMU.  The result would be
>> that guest-initiated DMA to the previously shared address would actually
>> work but would hit pages that are invisible to the guest.  And a whole
>> bunch of memory would be waste, but the whole system should stll work.
> 
> Do you mean to let VFIO & IOMMU to treat all guest memory as shared first,
> and then just allocate the private pages in another backing store? I guess
> that could work, but with the cost of allocating roughly 2x physical pages
> of the guest RAM size. After all, the shared pages shall be only a small
> part of guest memory.

Yes.

My point is that I don't think there should be any particular danger in
leaving the VFIO code alone as part of TDX enablement.  The code ought
to *work* even if it will be wildly inefficient.  If someone cares to
make it work better, they're welcome to do so.

--Andy