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

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Date:   Thu, 2 Sep 2021 12:07:54 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Joerg Roedel <jroedel@...e.de>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        David Hildenbrand <david@...hat.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>,
        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>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
 memory

On 9/2/21 11:57 AM, Sean Christopherson wrote:
> On Thu, Sep 02, 2021, Andy Lutomirski wrote:
>> On 9/2/21 2:27 AM, Joerg Roedel wrote:
>>> On Wed, Sep 01, 2021 at 09:07:59AM -0700, Andy Lutomirski wrote:
>>>> In principle, you could actually initialize a TDX guest with all of its
>>>> memory shared and all of it mapped in the host IOMMU.
>>> Not sure how this works in TDX, but in SEV code fetches are always
>>> treated as encrypted. So this approach would not work with SEV, not to
>>> speak about attestation, which will not work with this approach either
>>> :)
>>>
>> Oof.
> TDX is kinda similar.  _All_ accesses are private if paging is disabled because
> the shared bit is either bit 48 or bit 51 in the GPA, i.e. can't be reached if
> paging is disabled.  The vCPU is hardcoded to start in unpaged protected mode,
> so at least some amount of guest memory needs to be private.

That's a rule we should definitely add to our page table checker.  Just
like how we can look for W+X, we should also look for Shared+X.