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Date:   Wed, 01 Sep 2021 10:08:33 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "James Bottomley" <jejb@linux.ibm.com>,
        "David Hildenbrand" <david@redhat.com>,
        "Sean Christopherson" <seanjc@google.com>
Cc:     "Paolo Bonzini" <pbonzini@redhat.com>,
        "Vitaly Kuznetsov" <vkuznets@redhat.com>,
        "Wanpeng Li" <wanpengli@tencent.com>,
        "Jim Mattson" <jmattson@google.com>,
        "Joerg Roedel" <joro@8bytes.org>, "kvm list" <kvm@vger.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        "Borislav Petkov" <bp@alien8.de>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Joerg Roedel" <jroedel@suse.de>,
        "Andi Kleen" <ak@linux.intel.com>,
        "David Rientjes" <rientjes@google.com>,
        "Vlastimil Babka" <vbabka@suse.cz>,
        "Tom Lendacky" <thomas.lendacky@amd.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        "Ingo Molnar" <mingo@redhat.com>,
        "Varad Gautam" <varad.gautam@suse.com>,
        "Dario Faggioli" <dfaggioli@suse.com>,
        "the arch/x86 maintainers" <x86@kernel.org>, linux-mm@kvack.org,
        linux-coco@lists.linux.dev,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        "Sathyanarayanan Kuppuswamy" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        "Dave Hansen" <dave.hansen@intel.com>,
        "Yu Zhang" <yu.c.zhang@linux.intel.com>
Subject: =?UTF-8?Q?Re:_[RFC]_KVM:_mm:_fd-based_approach_for_supporting_KVM_guest_?=
 =?UTF-8?Q?private_memory?=
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On Wed, Sep 1, 2021, at 9:18 AM, James Bottomley wrote:
> On Wed, 2021-09-01 at 08:54 -0700, Andy Lutomirski wrote:
> [...]
> > If you want to swap a page on TDX, you can't.  Sorry, go directly to
> > jail, do not collect $200.
>=20
> Actually, even on SEV-ES you can't either.  You can read the encrypted
> page and write it out if you want, but unless you swap it back to the
> exact same physical memory location, the encryption key won't work.=20
> Since we don't guarantee this for swap, I think swap won't actually
> work for any confidential computing environment.
>=20
> > So I think there are literally zero code paths that currently call
> > try_to_unmap() that will actually work like that on TDX.  If we run
> > out of memory on a TDX host, we can kill the guest completely and
> > reclaim all of its memory (which probably also involves killing QEMU
> > or whatever other user program is in charge), but that's really our
> > only option.
>=20
> I think our only option for swap is guest co-operation.  We're going to
> have to inflate a balloon or something in the guest and have the guest
> driver do some type of bounce of the page, where it becomes an
> unencrypted page in the guest (so the host can read it without the
> physical address keying of the encryption getting in the way) but
> actually encrypted with a swap transfer key known only to the guest.  I
> assume we can use the page acceptance infrastructure currently being
> discussed elsewhere to do swap back in as well ... the host provides
> the guest with the encrypted swap page and the guest has to decrypt it
> and place it in encrypted guest memory.

I asked David, and he said the PSP offers a swapping mechanism for SEV-E=
S.  I haven=E2=80=99t read the details, but they should all be public.