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

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Message-ID: <YTEei9RBDHnRfe/B@google.com>
Date:   Thu, 2 Sep 2021 18:57:15 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     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>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest
 private memory

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.

I also could've sworn code fetches from shared memory would #VE, but I can't find
anything in the specs that confirm that.  I may be conflating TDX with SGX's #GP
on a code fetch outside of ELRANGE...