Date:   Wed, 13 Apr 2022 08:42:00 -0500
From:   Michael Roth <michael.roth@....com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Vishal Annapurve <vannapurve@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, <shauh@...nel.org>,
        <yang.zhong@...el.com>, <drjones@...hat.com>,
        <ricarkol@...gle.com>, <aaronlewis@...gle.com>,
        <wei.w.wang@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Quentin Perret <qperret@...gle.com>,
        Steven Price <steven.price@....com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Marc Orr <marcorr@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Peter Gonda <pgonda@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>, <diviness@...gle.com>
Subject: Re: [RFC V1 PATCH 0/5] selftests: KVM: selftests for fd-based
 approach of supporting private memory

On Tue, Apr 12, 2022 at 05:16:22PM -0700, Andy Lutomirski wrote:
> On Fri, Apr 8, 2022, at 2:05 PM, Vishal Annapurve wrote:
> > This series implements selftests targeting the feature floated by Chao
> > via:
> > https://lore.kernel.org/linux-mm/20220310140911.50924-1-chao.p.peng@linux.intel.com/
> >
> > Below changes aim to test the fd based approach for guest private memory
> > in context of normal (non-confidential) VMs executing on non-confidential
> > platforms.
> >
> > Confidential platforms along with the confidentiality aware software
> > stack support a notion of private/shared accesses from the confidential
> > VMs.
> > Generally, a bit in the GPA conveys the shared/private-ness of the
> > access. Non-confidential platforms don't have a notion of private or
> > shared accesses from the guest VMs. To support this notion,
> > KVM_HC_MAP_GPA_RANGE
> > is modified to allow marking an access from a VM within a GPA range as
> > always shared or private. Any suggestions regarding implementing this ioctl
> > alternatively/cleanly are appreciated.
> 
> This is fantastic.  I do think we need to decide how this should work in general.  We have a few platforms with somewhat different properties:
> 
> TDX: The guest decides, per memory access (using a GPA bit), whether an access is private or shared.  In principle, the same address could be *both* and be distinguished by only that bit, and the two addresses would refer to different pages.
> 
> SEV: The guest decides, per memory access (using a GPA bit), whether an access is private or shared.  At any given time, a physical address (with that bit masked off) can be private, shared, or invalid, but it can't be valid as private and shared at the same time.
> 
> pKVM (currently, as I understand it): the guest decides by hypercall, in advance of an access, which addresses are private and which are shared.
> 
> This series, if I understood it correctly, is like TDX except with no hardware security.
> 
> Sean or Chao, do you have a clear sense of whether the current fd-based private memory proposal can cleanly support SEV and pKVM?  What, if anything, needs to be done on the API side to get that working well?  I don't think we need to support SEV or pKVM right away to get this merged, but I do think we should understand how the API can map to them.

I've been looking at porting the SEV-SNP hypervisor patches over to
using memfd, and I hit an issue that I think is generally applicable
to SEV/SEV-ES as well. Namely at guest init time we have something
like the following flow:

  VMM:
    - allocate shared memory to back the guest and map it into guest
      address space
    - initialize shared memory with initialize memory contents (namely
      the BIOS)
    - ask KVM to encrypt these pages in-place and measure them to
      generate the initial measured payload for attestation, via
      KVM_SEV_LAUNCH_UPDATE with the GPA for each range of memory to
      encrypt.
  KVM:
    - issue SEV_LAUNCH_UPDATE firmware command, which takes an HPA as
      input and does an in-place encryption/measure of the page.

With current v5 of the memfd/UPM series, I think the expected flow is that
we would fallocate() these ranges from the private fd backend in advance of
calling KVM_SEV_LAUNCH_UPDATE (if VMM does it after we'd destroy the initial
guest payload, since they'd be replaced by newly-allocated pages). But if
VMM does it before, VMM has no way to initialize the guest memory contents,
since mmap()/pwrite() are disallowed due to MFD_INACCESSIBLE.

I think something similar to your proposal[1] here of making pread()/pwrite()
possible for private-fd-backed memory that's been flagged as "shareable"
would work for this case. Although here the "shareable" flag could be
removed immediately upon successful completion of the SEV_LAUNCH_UPDATE
firmware command.

I think with TDX this isn't an issue because their analagous TDH.MEM.PAGE.ADD
seamcall takes a pair of source/dest HPA as input params, so the VMM
wouldn't need write access to dest HPA at any point, just source HPA.

[1] https://lwn.net/ml/linux-kernel/eefc3c74-acca-419c-8947-726ce2458446@www.fastmail.com/

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