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

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Date:   Wed, 01 Sep 2021 09:31:49 -0700
From:   James Bottomley <jejb@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest
 private memory

On Wed, 2021-09-01 at 18:22 +0200, David Hildenbrand wrote:
> On 01.09.21 18:18, 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.
> > 
> > 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.  Since we don't guarantee this for swap, I think swap
> > won't actually work for any confidential computing environment.
> > 
> > > 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.
> > 
> > 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.
> 
> Ballooning is indeed *the* mechanism to avoid swapping in the
> hypervisor  and much rather let the guest swap. Shame it requires
> trusting a guest, which we, in general, can't. Not to mention other
> issues we already do have with ballooning (latency, broken auto-
> ballooning, over-inflating, ...).


Well not necessarily, but it depends how clever we want to get.  If you
look over on the OVMF/edk2 list, there's a proposal to do guest
migration via a mirror VM that invokes a co-routine embedded in the
OVMF binary:

https://patchew.org/EDK2/20210818212048.162626-1-tobin@linux.ibm.com/

This gives us a page encryption mechanism that's provided by the host
but accepted via the guest using attestation, meaning we have a
mutually trusted piece of code that can use to extract encrypted pages.
It does seem it could be enhanced to do swapping for us as well if
that's a road we want to go down?

James